Staphylococcus aureus

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Staphylococcus aureus
Staphylococcus aureus VISA 2.jpg
Scanning electron micrograph of S. aureus; false color added
Scientific classification
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S. aureus
Binomial name
Staphylococcus aureus

Staphylococcus aureus is a Gram-positive, round-shaped bacterium that is a member of the Firmicutes, and it is a usual member of the microbiota of the body, frequently found in the upper respiratory tract and on the skin. It is often positive for catalase and nitrate reduction and is a facultative anaerobe that can grow without the need for oxygen.[1] Although S. aureus usually acts as a commensal of the human microbiota it can also become an opportunistic pathogen, being a common cause of skin infections including abscesses, respiratory infections such as sinusitis, and food poisoning. Pathogenic strains often promote infections by producing virulence factors such as potent protein toxins, and the expression of a cell-surface protein that binds and inactivates antibodies. The emergence of antibiotic-resistant strains of S. aureus such as methicillin-resistant S. aureus (MRSA) is a worldwide problem in clinical medicine. Despite much research and development, no vaccine for S. aureus has been approved.

An estimated 20% to 30% of the human population are long-term carriers of S. aureus[2][3] which can be found as part of the normal skin flora, in the nostrils,[2][4] and as a normal inhabitant of the lower reproductive tract of women.[5][6] S. aureus can cause a range of illnesses, from minor skin infections, such as pimples,[7] impetigo, boils, cellulitis, folliculitis, carbuncles, scalded skin syndrome, and abscesses, to life-threatening diseases such as pneumonia, meningitis, osteomyelitis, endocarditis, toxic shock syndrome, bacteremia, and sepsis. It is still one of the five most common causes of hospital-acquired infections and is often the cause of wound infections following surgery. Each year, around 500,000 patients in hospitals of the United States contract a staphylococcal infection, chiefly by S. aureus.[8] Up to 50,000 deaths each year in the USA are linked with S. aureus infections.[9]

Staphylococcus aureus
Classification and external resources
SpecialtyInfectious disease
ICD-9-CM041.11

History[edit]

Discovery[edit]

1881, Sir Alexander Ogston, a Scottish surgeon, discovered that Staphylococcus can cause wound infections after noticing groups of bacteria in pus from a surgical abscess during a procedure he was performing. He named it Staphylococcus after its clustered appearance evident under a microscope. Then, in 1884, German scientist Friedrich Julius Rosenbach identified Staphylococcus aureus, discriminating and separating it from Staphylococcus albus, a related bacterium. In the early 1930s, doctors began to use a more streamlined test to detect the presence of an S. aureus infection by the means of coagulase testing, which enables detection of an enzyme produced by the bacterium. Prior to the 1940s, S. aureus infections were fatal in the majority of patients. However, doctors discovered that the use of penicillin could cure S. aureus infections. Unfortunately, by the end of the 1940s, penicillin resistance became widespread amongst this bacterium population and outbreaks of the resistant strain began to occur[10].

Evolution[edit]

Staphylococcus aureus can be sorted into ten dominant human lineages. There are numerous minor lineages as well, but these are not seen in the population as often. Genomes of bacteria within the same lineage are mostly conserved, with the exception of mobile genetic elements. Mobile genetic elements that are common in S. aureus include bacteriophages, pathogenicity islands, plasmids, transposons, and staphylococcal cassette chromosomes. These elements have enabled S. aureus to continually evolve and gain new traits. There is a great deal of genetic variation within the S. aureus species. A study by Fitzgerald et al. (2001) revealed that approximately 22% of the S. aureus genome is non-coding and thus can differ from bacterium to bacterium. An example of this difference is seen in the species’ virulence. Only a few strains of S. aureus are associated with infections in humans. This demonstrates that there is a large range of infectious ability within the species[11].

It has been proposed that one possible reason for the great deal of heterogeneity within the species could be due to its reliance on heterogeneous infections. This occurs when multiple different types of S. aureus cause an infection within a host. The different strains can secrete different enzymes or bring different antibiotic resistances to the group, increasing its pathogenic ability[12]. Thus, there is a need for a large number of mutations and acquisitions of mobile genetic elements.

Another notable evolutionary process within the S. aureus species is its co-evolution with its human hosts. Overtime, this parasitic relationship has led to the bacterium's ability to be carried in the nasopharynx of humans without causing symptoms or infection. This allows it to be passed throughout the human population, increasing its fitness as a species[13]. However, only approximately 50% of the human population are carriers of S. aureus, with 20% as continuous carriers and 30% as intermittent. This leads scientists to believe that there are many factors that determine whether S. aureus is carried asymptomatically in humans, including factors that are specific to an individual person. According to a 1995 study by Hofman et al., these factors may include age, sex, diabetes, and smoking. They also determined some genetic variations in humans that lead to an increased ability for S. aureus to colonize, notably a polymorphism in the glucocorticoid receptor gene that results in larger corticoid production. In conclusion, there is evidence that any strain of this bacterium can become invasive, as this is highly dependent upon human factors[14].

Though S. aureus has quick reproductive and micro-evolutionary rates, there are multiple barriers that prevent evolution with the species. One such barrier is AGR, which is a global accessory gene regulator within the bacteria. This such regulator has been linked to the virulence level of the bacteria. Loss of function mutations within this gene have been found to increase the fitness of the bacterium containing it. Thus, S. aureus must make a trade-off to increase their success as a species, exchanging reduced virulence for increased drug resistance. Another barrier to evolution is the Sau1 Type I restriction modification (RM) system. This system exists to protect the bacterium from foreign DNA by digesting it. Exchange of DNA between the same lineage is not blocked, since they have the same enzymes and the RM system does not recognize the new DNA as foreign, but transfer between lineages is blocked[12].

Microbiology[edit]

Gram stain of S. saprophyticus cells which typically occur in clusters: The cell wall readily absorbs the crystal violet stain.
Yellow colonies of S. aureus on a blood agar plate, note regions of clearing around colonies caused by lysis of red cells in the agar (beta hemolysis)

S. aureus (/ˌstæfɪləˈkɒkəs ˈɔːriəs, -l-/,[15][16] Greek σταφυλόκοκκος, "grape-cluster berry", Latin aureus, "golden") is a facultative anaerobic, Gram-positive coccal (round) bacterium also known as "golden staph" and "oro staphira". S. aureus is nonmotile and does not form spores.[17] In medical literature, the bacterium is often referred to as S. aureus, Staph aureus or Staph a..[18] S. aureus appears as staphylococci (grape-like clusters) when viewed through a microscope, and has large, round, golden-yellow colonies, often with hemolysis, when grown on blood agar plates.[19] S. aureus reproduces asexually by binary fission. Complete separation of the daughter cells is mediated by S. aureus autolysin, and in its absence or targeted inhibition, the daughter cells remain attached to one another and appear as clusters.[20]

S. aureus is catalase-positive (meaning it can produce the enzyme catalase). Catalase converts hydrogen peroxide (H
2
O
2
) to water and oxygen. Catalase-activity tests are sometimes used to distinguish staphylococci from enterococci and streptococci. Previously, S. aureus was differentiated from other staphylococci by the coagulase test. However, not all S. aureus strains are coagulase-positive[19][21] and incorrect species identification can impact effective treatment and control measures.[22]

Staphylococcus is different from the similarly named and medically relevant genus Streptococcus.

Natural genetic transformation is a reproductive process involving DNA transfer from one bacterium to another through the intervening medium, and the integration of the donor sequence into the recipient genome by homologous recombination. S. aureus was found to be capable of natural genetic transformation, but only at low frequency under the experimental conditions employed.[23] Further studies suggested that the development of competence for natural genetic transformation may be substantially higher under appropriate conditions, yet to be discovered.[24]

Role in health[edit]

In humans, S. aureus is not part of the normal microbiota present in the upper respiratory tract, rather it is a colonization [25] and on skin and in the gut mucosa.[26]

S. aureus, along with similar species that can colonize and act symbiotically but can cause disease if they begin to take over the tissues they have colonized or invade other tissues, have been called "pathobionts".[25]

Role in disease[edit]

This 2005 scanning electron micrograph (SEM) depicts numerous clumps of methicillin-resistant S. aureus (MRSA) bacteria.

While S. aureus usually acts as a commensal bacterium, asymptomatically colonizing about 30% of the human population, it can sometimes cause disease.[3] In particular, S. aureus is one of the most common causes of bacteremia and infective endocarditis. Additionally, it can cause various skin and soft-tissue infections,[3] particularly when skin or mucosal barriers have been breached.

S. aureus infections can spread through contact with pus from an infected wound, skin-to-skin contact with an infected person, and contact with objects used by an infected person such as towels, sheets, clothing, or athletic equipment. Joint replacements put a person at particular risk of septic arthritis, staphylococcal endocarditis (infection of the heart valves), and pneumonia.[27]

Diabetics, injection drug users, and individuals with heart conditions should take extra precautions to avoid coming into contact with S. aureus, as they are at the highest risk. Preventive measures include washing hands often with soap and making sure to bathe or shower daily.

S. aureus is a significant cause of chronic biofilm infections on medical implants, and the repressor of toxins is part of the infection pathway.[28]

S. aureus can lay dormant in the body for years undetected. Once symptoms begin to show, the host is contagious for another two weeks, and the overall illness lasts a few weeks. If untreated, though, the disease can be deadly.[29] Deeply penetrating S. aureus infections can be severe.

Skin infections[edit]

Skin infections are the most common form of S. aureus infection. This can manifest in various ways, including small benign boils, folliculitis, impetigo, cellulitis, and more severe, invasive soft-tissue infections.[7][3]

S. aureus is extremely prevalent in persons with atopic dermatitis, more commonly known as eczema. It is mostly found in fertile, active places, including the armpits, hair, and scalp. Large pimples that appear in those areas may exacerbate the infection if lacerated. This can lead to staphylococcal scalded skin syndrome, a severe form of which can be seen in newborns.[30]

The presence of S. aureus in persons with atopic dermatitis is not an indication to treat with oral antibiotics, as evidence has not shown this to give benefit to the patient.[31][32] However, topical antibiotics combined with corticosteroids have been found to improve the condition.[33] Colonization of S. aureus drives inflammation of atopic dermatitis;[34] S. aureus is believed to exploit defects in the skin barrier of persons with atopic dermatitis, triggering cytokine expression and therefore exacerbating symptoms.[35]

Food poisoning[edit]

S. aureus is also responsible for food poisoning. It is capable of generating toxins that produce food poisoning in the human body.[36] Its incubation period lasts one to six hours,[37] with the illness itself lasting from 30 minutes to 3 days.[38] Preventive measures one can take to help prevent the spread of the disease include washing hands thoroughly with soap and water before preparing food. Stay away from any food if ill, and wear gloves if any open wounds occur on hands or wrists while preparing food. If storing food for longer than 2 hours, keep the food above 140 or below 40 °F.[39]

Bone and joint infections[edit]

S. aureus is the bacterium commonly responsible for all major bone and joint infections. This manifests in one of three forms: osteomyelitis, septic arthritis, and infection from a replacement joint surgery.[40][3]

Bacteremia[edit]

S. aureus is a leading cause of bloodstream infections throughout much of the industrialized world.[40] Infection is generally associated with breaks in the skin or mucosal membranes due to surgery, injury, or use of intravascular devices such as catheters, hemodialysis machines, or injected drugs.[3][40] Once the bacteria have entered the bloodstream, they can infect various organs, causing infective endocarditis, septic arthritis, and osteomyelitis.[40] This disease is particularly prevalent and severe in the very young and very old.[3]

Without antibiotic treatment, S. aureus bacteremia has a case fatality rate around 80%.[3] With antibiotic treatment, case fatality rates range from 15% to 50% depending on the age and health of the patient, as well as the antibiotic resistance of the S. aureus strain.[3]

Medical implant infections[edit]

S. aureus is often found in biofilms formed on medical devices implanted in the body or on human tissue. It is commonly found with another pathogen, Candida albicans, forming multispecies biofilms. The latter is suspected to help S. aureus penetrate human tissue.[9] A higher mortality is linked with multispecies biofilms.[41]

S. aureus biofilm is the predominant cause of orthopedic implant-related infections, but is also found on cardiac implants, vascular grafts, various catheters, and cosmetic surgical implants.[42][43] After implantation, the surface of these devices becomes coated with host proteins, which provide a rich surface for bacterial attachment and biofilm formation. Once the device becomes infected, it must by completely removed, since S. aureus biofilm cannot be destroyed by antibiotic treatments.[43]

Current therapy for S. aureus biofilm-mediated infections involves surgical removal of the infected device followed by antibiotic treatment. Conventional antibiotic treatment alone is not effective in eradicating such infections.[42] An alternative to postsurgical antibiotic treatment is using antibiotic-loaded, dissolvable calcium sulfate beads, which are implanted with the medical device. These beads can release high doses of antibiotics at the desired site to prevent the initial infection.[43]

Novel treatments for S. aureus biofilm involving nano silver particles, bacteriophages, and plant-derived antibiotic agents are being studied. These agents have shown inhibitory effects against S. aureus embedded in biofilms.[44] A class of enzymes have been found to have biofilm matrix-degrading ability, thus may be used as biofilm dispersal agents in combination with antibiotics.[45]

Animal infections[edit]

S. aureus can survive on dogs,[46] cats,[47] and horses,[48] and can cause bumblefoot in chickens.[49] Some believe health-care workers' dogs should be considered a significant source of antibiotic-resistant S. aureus, especially in times of outbreak.[46] In a 2008 study by Boost, O’Donoghue, and James, it was found that just about 90% of S. aureus colonized within pet dogs presented as resistant to at least one antibiotic. The nasal region has been implicated as the most important site of transfer between dogs and humans[50].

S. aureus is one of the causal agents of mastitis in dairy cows. Its large polysaccharide capsule protects the organism from recognition by the cow's immune defenses.[51]

Virulence factors[edit]

Enzymes[edit]

S. aureus produces various enzymes such as coagulase (bound and free coagulases) which clots plasma and coats the bacterial cell, probably to prevent phagocytosis. Hyaluronidase (also known as spreading factor) breaks down hyaluronic acid and helps in spreading it. S. aureus also produces deoxyribonuclease, which breaks down the DNA, lipase to digest lipids, staphylokinase to dissolve fibrin and aid in spread, and beta-lactamase for drug resistance.[52]

Toxins[edit]

Depending on the strain, S. aureus is capable of secreting several exotoxins, which can be categorized into three groups. Many of these toxins are associated with specific diseases.[53]

Superantigens
Antigens known as superantigens can induce toxic shock syndrome (TSS). This group includes the toxins TSST-1, and enterotoxin type B, which causes TSS associated with tampon use. Toxic shock syndrome is characterized by fever, erythematous rash, low blood pressure, shock, multiple organ failure, and skin peeling. Lack of antibody to TSST-1 plays a part in the pathogenesis of TSS. Other strains of S. aureus can produce an enterotoxin that is the causative agent of a type of gastroenteritis. This form of gastroenteritis is self-limiting, characterized by vomiting and diarrhea 1-6 hours after ingestion of the toxin, with recovery in 8 to 24 hours. Symptoms include nausea, vomiting, diarrhea, and major abdominal pain.[54][55]

Exfoliative toxins
Exfoliative toxins are exotoxins implicated in the disease staphylococcal scalded skin syndrome (SSSS), which occurs most commonly in infants and young children. It also may occur as epidemics in hospital nurseries. The protease activity of the exfoliative toxins causes peeling of the skin observed with SSSS.[55]
Other toxins
Staphylococcal toxins that act on cell membranes include alpha toxin, beta toxin, delta toxin, and several bicomponent toxins. Strains of S. aureus can host phages, such as the prophage Φ-PVL that produces Panton-Valentine leukocidin (PVL), to increase virulence. The bicomponent toxin PVL is associated with severe necrotizing pneumonia in children.[56][57] The genes encoding the components of PVL are encoded on a bacteriophage found in community-associated MRSA strains.[citation needed]

small RNA[edit]

The list of small RNAs involved in the control of bacterial virulence in S.aureus is growing. This can be facilitated by factors such as increased biofilm formation in the presence of increased levels of such small RNAs[58]. For example, RNAIII,[59] SprD,[60] SprC,[61][62] RsaE,[63] SprA1,[64] SSR42,[65] ArtR,[66] SprX and Teg49.[67]

Strategies for post-transcriptional regulation by 3'untranslated region[edit]

Many mRNAs in S. aureus carry three prime untranslated regions (3'UTR) longer than 100 nucleotides, which may potentially have a regulatory function.[68]

Further investigation of icaR mRNA (mRNA coding for the repressor of the main expolysaccharidic compound of the bacteria biofilm matrix) demonstrated that the 3'UTR binding to the 5' UTR can interfere with the translation initiation complex and generate a double stranded substrate for RNase III. The interaction is between the UCCCCUG motif in the 3'UTR and the Shine-Dalagarno region at the 5'UTR. Deletion of the motif resulted in IcaR repressor accumulation and inhibition of biofilm development.[68] The biofilm formation is the main cause of Staphylococcus implant infections.[69]

Biofilm[edit]

Bioflims are groups of microorganisms, such as bacteria, that attach to each other and grow on wet surfaces[70]. S. aureus biofilm has high resistance to antibiotic treatments and host immune response[71]. One hypothesis for explaining this is that the biofilm matrix protects the embedded cells by acting as a barrier to prevent antibiotic penetration. However, the biofilm matrix is composed with many water channels, so this hypothesis is becoming increasingly less likely, but a biofilm matrix possibly contains antibiotic‐degrading enzymes such as β-lactamases, which can prevent antibiotic penetration.[72] Another hypothesis is that the conditions in the biofilm matrix favor the formation of persister cells, which are highly antibiotic-resistant, dormant bacterial cells.[43] S. aureus biofilms also have high resistance to host immune response. Though the exact mechanism of resistance is unknown, S. aureus biofilms have increased growth under the presence of cytokines produced by the host immune response.[73] Host antibodies are less effective for S. aureus biofilm due to the heterogeneous antigen distribution, where an antigen may be present in some areas of the biofilm, but completely absent from other areas.[43]

Other immunoevasive strategies[edit]

Protein A

Protein A is anchored to staphylococcal peptidoglycan pentaglycine bridges (chains of five glycine residues) by the transpeptidase sortase A.[74] Protein A, an IgG-binding protein, binds to the Fc region of an antibody. In fact, studies involving mutation of genes coding for protein A resulted in a lowered virulence of S. aureus as measured by survival in blood, which has led to speculation that protein A-contributed virulence requires binding of antibody Fc regions.[75]

Protein A in various recombinant forms has been used for decades to bind and purify a wide range of antibodies by immunoaffinity chromatography. Transpeptidases, such as the sortases responsible for anchoring factors like protein A to the staphylococcal peptidoglycan, are being studied in hopes of developing new antibiotics to target MRSA infections.[76]

S. aureus on trypticase soy agar: The strain is producing a yellow pigment staphyloxanthin.
Staphylococcal pigments

Some strains of S. aureus are capable of producing staphyloxanthin — a golden-coloured carotenoid pigment. This pigment acts as a virulence factor, primarily by being a bacterial antioxidant which helps the microbe evade the reactive oxygen species which the host immune system uses to kill pathogens.[77][78]

Mutant strains of S. aureus modified to lack staphyloxanthin are less likely to survive incubation with an oxidizing chemical, such as hydrogen peroxide, than pigmented strains. Mutant colonies are quickly killed when exposed to human neutrophils, while many of the pigmented colonies survive.[77] In mice, the pigmented strains cause lingering abscesses when inoculated into wounds, whereas wounds infected with the unpigmented strains quickly heal.

These tests suggest the Staphylococcus strains use staphyloxanthin as a defence against the normal human immune system. Drugs designed to inhibit the production of staphyloxanthin may weaken the bacterium and renew its susceptibility to antibiotics.[78] In fact, because of similarities in the pathways for biosynthesis of staphyloxanthin and human cholesterol, a drug developed in the context of cholesterol-lowering therapy was shown to block S. aureus pigmentation and disease progression in a mouse infection model.[79]

Classical diagnosis[edit]

Typical Gram-positive cocci, in clusters, from a sputum sample, Gram stain

Depending upon the type of infection present, an appropriate specimen is obtained accordingly and sent to the laboratory for definitive identification by using biochemical or enzyme-based tests. A Gram stain is first performed to guide the way, which should show typical Gram-positive bacteria, cocci, in clusters. Second, the isolate is cultured on mannitol salt agar, which is a selective medium with 7–9% NaCl that allows S. aureus to grow, producing yellow-colored colonies as a result of mannitol fermentation and subsequent drop in the medium's pH.[citation needed]

Furthermore, for differentiation on the species level, catalase (positive for all Staphylococcus species), coagulase (fibrin clot formation, positive for S. aureus), DNAse (zone of clearance on DNase agar), lipase (a yellow color and rancid odor smell), and phosphatase (a pink color) tests are all done. For staphylococcal food poisoning, phage typing can be performed to determine whether the staphylococci recovered from the food were the source of infection.[citation needed]

Rapid diagnosis and typing[edit]

Recent activities and food that a patient has recently eaten will be inquired about by a physician, and a physical examination is conducted to review any symptoms. With more severe symptoms, blood tests and stool culture may be in order.[80] Diagnostic microbiology laboratories and reference laboratories are key for identifying outbreaks and new strains of S. aureus. Recent genetic advances have enabled reliable and rapid techniques for the identification and characterization of clinical isolates of S. aureus in real time. These tools support infection control strategies to limit bacterial spread and ensure the appropriate use of antibiotics. Quantitative PCR is increasingly being used to identify outbreaks of infection.[81][82]

When observing the evolvement of S. aureus and its ability to adapt to each modified antibiotic, two basic methods known as “band-based” or “sequence-based” are employed.[83] Keeping these two methods in mind, other methods such as multilocus sequence typing (MLST), pulsed-field gel electrophoresis (PFGE), bacteriophage typing, spa locus typing, and SCCmec typing are often conducted more than others.[84] With these methods, it can be determined where strains of MRSA originated and also where they are currently.[85]

With MLST, this technique of typing uses fragments of several housekeeping genes known as aroE, glpF, gmk, pta, tip, and yqiL. These sequences are then assigned a number which give to a string of several numbers that serve as the allelic profile. Although this is a common method, a limitation about this method is the maintenance of the microarray which detects newly allelic profiles, making it a costly and time-consuming experiment.[83]

With PFGE, a method which is still very much used dating back to its first success in 1980s, remains capable of helping differentiate MRSA isolates.[85] To accomplish this, the technique uses multiple gel electrophoresis, along with a voltage gradient to display clear resolutions of molecules. The S. aureus fragments then transition down the gel, producing specific band patterns that are later compared with other isolates in hopes of identifying related strains. Limitations of the method include practical difficulties with uniform band patterns and PFGE sensitivity as a whole.

Spa locus typing is also considered a popular technique that uses a single locus zone in a polymorphic region of S. aureus to distinguish any form of mutations.[85] Although this technique is often inexpensive and less time-consuming, the chance of losing discriminatory power makes it hard to differentiate between MLST CCs exemplifies a crucial limitation.

Treatment[edit]

The treatment of choice for S. aureus infection is penicillin. An antibiotic derived from some Penicillium fungal species, penicillin inhibits the formation of peptidoglycan cross-linkages that provide the rigidity and strength in a bacterial cell wall. The four-membered β-lactam ring of penicillin is bound to enzyme DD-transpeptidase, an enzyme that when functional, cross-links chains of peptidoglycan that form bacterial cell walls. The binding of β-lactam to DD-transpeptidase inhibits the enzyme's functionality and it can no longer catalyze the formation of the cross-links. As a result, cell wall formation and degradation are imbalanced, thus resulting in cell death. In most countries, however, penicillin resistance is extremely common, and first-line therapy is most commonly a penicillinase-resistant β-lactam antibiotic (for example, oxacillin or flucloxacillin, both of which have the same mechanism of action as penicillin). Combination therapy with gentamicin may be used to treat serious infections, such as endocarditis,[86][87] but its use is controversial because of the high risk of damage to the kidneys.[88] The duration of treatment depends on the site of infection and on severity. Adjunctive rifampicin has been historically used in the management of S aureus bacteraemia, but randomised controlled trial evidence has shown this to be of no overall benefit over standard antibiotic therapy.[89]

Antibiotic resistance in S. aureus was uncommon when penicillin was first introduced in 1943. Indeed, the original Petri dish on which Alexander Fleming of Imperial College London observed the antibacterial activity of the Penicillium fungus was growing a culture of S. aureus. By 1950, 40% of hospital S. aureus isolates were penicillin-resistant; by 1960, this had risen to 80%.[90]

MRSA, often pronounced /ˈmɜːrsə/ or /ɛm ɑːr ɛs /, is one of a number of greatly feared strains of S. aureus which have become resistant to most β-lactam antibiotics. For this reason, vancomycin, a glycopeptide antibiotic, is commonly used to combat MRSA. Vancomycin inhibits the synthesis of peptidoglycan, but unlike β-lactam antibiotics, glycopeptide antibiotics target and bind to amino acids in the cell wall, preventing peptidoglycan cross-linkages from forming. MRSA strains are most often found associated with institutions such as hospitals, but are becoming increasingly prevalent in community-acquired infections.

Minor skin infections can be treated with triple antibiotic ointment.[91]

Antibiotic resistance[edit]

Bacterial cells of S. aureus, which is one of the causal agents of mastitis in dairy cows: Its large capsule protects the organism from attack by the cow's immunological defenses.

Staphylococcal resistance to penicillin is mediated by penicillinase (a form of β-lactamase) production: an enzyme that cleaves the β-lactam ring of the penicillin molecule, rendering the antibiotic ineffective. Penicillinase-resistant β-lactam antibiotics, such as methicillin, nafcillin, oxacillin, cloxacillin, dicloxacillin, and flucloxacillin, are able to resist degradation by staphylococcal penicillinase.

Resistance to methicillin is mediated via the mec operon, part of the staphylococcal cassette chromosome mec (SCCmec). SCCmec is a family of mobile genetic elements, which is a major driving force of S. aureus evolution[92]. Resistance is conferred by the mecA gene, which codes for an altered penicillin-binding protein (PBP2a or PBP2') that has a lower affinity for binding β-lactams (penicillins, cephalosporins, and carbapenems). This allows for resistance to all β-lactam antibiotics, and obviates their clinical use during MRSA infections. Studies have explained that this mobile genetic element has been acquired by different lineages in separate gene transfer events, indicating that there is not a common ancestor of differing MRSA strains. For example, community- associated MRSA (CA-MRSA) and livestock-associated MRSA (LA-MRSA) were found to have originated and progressed independently from MRSA found in hospitals[93]. As such, the glycopeptide vancomycin is often deployed against MRSA. In addition, linezolid-resistant Staphylococcus aureus was discovered in 2001. This strain was found to contain mutations in the gene coding for ribosomal methyltransferase[93].

Aminoglycoside antibiotics, such as kanamycin, gentamicin, streptomycin, etc., were once effective against staphylococcal infections until strains evolved mechanisms to inhibit the aminoglycosides' action, which occurs via protonated amine and/or hydroxyl interactions with the ribosomal RNA of the bacterial 30S ribosomal subunit.[94] Three main mechanisms of aminoglycoside resistance mechanisms are currently and widely accepted: aminoglycoside modifying enzymes, ribosomal mutations, and active efflux of the drug out of the bacteria.

Aminoglycoside-modifying enzymes inactivate the aminoglycoside by covalently attaching either a phosphate, nucleotide, or acetyl moiety to either the amine or the alcohol key functional group (or both groups) of the antibiotic. This changes the charge or sterically hinders the antibiotic, decreasing its ribosomal binding affinity. In S. aureus, the best-characterized aminoglycoside-modifying enzyme is aminoglycoside adenylyltransferase 4' IA (ANT(4')IA). This enzyme has been solved by X-ray crystallography.[95] The enzyme is able to attach an adenyl moiety to the 4' hydroxyl group of many aminoglycosides, including kamamycin and gentamicin.

Glycopeptide resistance is mediated by acquisition of the vanA gene, which originates from the Tn1546 transposon found in a plasmid in[93] enterococci and codes for an enzyme that produces an alternative peptidoglycan to which vancomycin will not bind.

Today, S. aureus has become resistant to many commonly used antibiotics. In the UK, only 2% of all S. aureus isolates are sensitive to penicillin, with a similar picture in the rest of the world. The β-lactamase-resistant penicillins (methicillin, oxacillin, cloxacillin, and flucloxacillin) were developed to treat penicillin-resistant S. aureus, and are still used as first-line treatment. Methicillin was the first antibiotic in this class to be used (it was introduced in 1959), but, only two years later, the first case of methicillin-resistant Staphylococcus aureus (MRSA) was reported in England.[96]

Despite this, MRSA generally remained an uncommon finding, even in hospital settings, until the 1990s, when the MRSA prevalence in hospitals exploded, and it is now endemic.[97]

MRSA infections in both the hospital and community setting are commonly treated with non-β-lactam antibiotics, such as clindamycin (a lincosamine) and co-trimoxazole (also commonly known as trimethoprim/sulfamethoxazole). Resistance to these antibiotics has also led to the use of new, broad-spectrum anti-Gram-positive antibiotics, such as linezolid, because of its availability as an oral drug. First-line treatment for serious invasive infections due to MRSA is currently glycopeptide antibiotics (vancomycin and teicoplanin). A number of problems with these antibiotics occur, such as the need for intravenous administration (no oral preparation is available), toxicity, and the need to monitor drug levels regularly by blood tests. Also, glycopeptide antibiotics do not penetrate very well into infected tissues (this is a particular concern with infections of the brain and meninges and in endocarditis). Glycopeptides must not be used to treat methicillin-sensitive S. aureus (MSSA), as outcomes are inferior.[98]

Because of the high level of resistance to penicillins and because of the potential for MRSA to develop resistance to vancomycin, the U.S. Centers for Disease Control and Prevention has published guidelines for the appropriate use of vancomycin. In situations where the incidence of MRSA infections is known to be high, the attending physician may choose to use a glycopeptide antibiotic until the identity of the infecting organism is known. After the infection is confirmed to be due to a methicillin-susceptible strain of S. aureus, treatment can be changed to flucloxacillin or even penicillin], as appropriate.

Vancomycin-resistant S. aureus (VRSA) is a strain of S. aureus that has become resistant to the glycopeptides. The first case of vancomycin-intermediate S. aureus (VISA) was reported in Japan in 1996;[99] but the first case of S. aureus truly resistant to glycopeptide antibiotics was only reported in 2002.[100] Three cases of VRSA infection had been reported in the United States as of 2005.[101]

Carriage[edit]

About 33% of the U.S. population are carriers of S. aureus and about 2% carry MRSA.[102]

The carriage of S. aureus is an important source of hospital-acquired infection (also called nosocomial) and community-acquired MRSA. Although S. aureus can be present on the skin of the host, a large proportion of its carriage is through the anterior nares of the nasal passages[2] and can further be present in the ears.[103] The ability of the nasal passages to harbour S. aureus results from a combination of a weakened or defective host immunity and the bacterium's ability to evade host innate immunity.[104] Nasal carriage is also implicated in the occurrence of staph infections.[105]

Infection control[edit]

Spread of S. aureus (including MRSA) generally is through human-to-human contact, although recently some veterinarians have discovered the infection can be spread through pets,[106] with environmental contamination thought to play a relatively unimportant part. Emphasis on basic hand washing techniques are, therefore, effective in preventing its transmission. The use of disposable aprons and gloves by staff reduces skin-to-skin contact, so further reduces the risk of transmission.

Recently, myriad cases of S. aureus have been reported in hospitals across America. Transmission of the pathogen is facilitated in medical settings where healthcare worker hygiene is insufficient. S. aureus is an incredibly hardy bacterium, as was shown in a study where it survived on polyester for just under three months;[107] polyester is the main material used in hospital privacy curtains.

The bacteria are transported on the hands of healthcare workers, who may pick them up from a seemingly healthy patient carrying a benign or commensal strain of S. aureus, and then pass it on to the next patient being treated. Introduction of the bacteria into the bloodstream can lead to various complications, including endocarditis, meningitis, and, if it is widespread, sepsis.

Ethanol has proven to be an effective topical sanitizer against MRSA. Quaternary ammonium can be used in conjunction with ethanol to increase the duration of the sanitizing action. The prevention of nosocomial infections involves routine and terminal cleaning. Nonflammable alcohol vapor in CO
2
NAV-CO2 systems have an advantage, as they do not attack metals or plastics used in medical environments, and do not contribute to antibacterial resistance.

An important and previously unrecognized means of community-associated MRSA colonization and transmission is during sexual contact.[108]

S. aureus is killed in one minute at 78 °C and in ten minutes at 64 °C.[109]

Certain strains of S. aureus have been described as being resistant to chlorine disinfection[110][111]

Top common bacterium in each industry
Catering industry
Vibrio parahaemolyticus, S. aureus, Bacillus cereus
Medical industry
Escherichia coli, S. aureus, Pseudomonas aeruginosa[112]

Research[edit]

As of 2015, no approved vaccine exists against S. aureus. Early clinical trials have been conducted for several vaccines candidates such as Nabi's StaphVax and PentaStaph, Intercell's / Merck's V710, VRi's SA75, and others.[113]

While some of these vaccines candidates have shown immune responses, other aggravated an infection by S. aureus. To date, none of these candidates provides protection against a S. aureus infection. The development of Nabi's StaphVax was stopped in 2005 after phase III trials failed.[114] Intercell's first V710 vaccine variant was terminated during phase II/III after higher mortality and morbidity were observed among patients who developed S. aureus infection.[115]

Nabi's enhanced S. aureus vaccines candidate PentaStaph was sold in 2011 to GlaxoSmithKline Biologicals S.A.[116] The current status of PentaStaph is unclear. A WHO document indicates that PentaStaph is failed in phase III trial stage.[117]

In 2010, GlaxoSmithKline started a phase 1 blind study to evaluate its GSK2392103A vaccine.[118] As of 2016, this vaccine is no longer under active development.[119]

Pfizer's S. aureus four-antigen vaccine SA4Ag was granted fast track designation by the U.S. Food and Drug Administration in February 2014.[120] In 2015, Pfizer has commenced a phase 2b trial regarding the SA4Ag vaccine.[121] Phase 1 results published in February 2017 showed a very robust and secure immunogenicity of SA4Ag.[122]

Novartis Vaccines and Diagnostics, a former division of Novartis and now part of GlaxoSmithKline, published in 2015 promising pre-clinical results of their four-component Staphylococcus aureus vaccine, 4C-staph.[123]

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