Staphylococcus Aureus PDF

# Section III Bacteriology ## Staphylococcus Aureus ### Morphology and Staining * Gram-positive cocci in pairs, tetrads, and clusters. * Original magnification x1000. ### Susceptibility to Antimicrobial Drugs **Staphylococci are variably susceptible to many antimicrobial drugs.** **Resistan...

# Section III Bacteriology ## Staphylococcus Aureus ### Morphology and Staining * Gram-positive cocci in pairs, tetrads, and clusters. * Original magnification x1000. ### Susceptibility to Antimicrobial Drugs **Staphylococci are variably susceptible to many antimicrobial drugs.** **Resistance is caused by several mechanisms**: - **β-Lactamase production:** common, is under plasmid control and makes the organisms resistant to many penicillins (penicillin G, ampicillin, piperacillin, and similar drugs). The plasmids are transmitted by transduction and perhaps also by conjugation. - **Resistance to nafcillin:** (and to methicillin and oxacillin) is independent of β-lactamase production. Resistance to nafcillin is encoded and regulated by a sequence of genes found in a region of the chromosome called the staphylococcal cassette chromosome mec (SCCmec). There are 12 different SCCmec types. - Types I, II, III, VI, and VIII are associated with hospital-acquired methicillin-resistant S. aureus (HA-MRSA) infections. - Type IV has been found in CA-MRSA strains. - Types IX and X are associated with animals (livestock-associated MRSA [LA-MRSA]). - **Susceptibility to vancomycin:** - Susceptible: minimum inhibitory concentration (MIC) is 2 µg/mL or less. - Intermediate susceptibility: MIC is 4-8 µg/mL. - Resistant: MIC is 16 µg/mL or greater. - Strains of S. aureus with intermediate susceptibility to vancomycin have been isolated in Japan, the United States, and several other countries. These are often known as vancomycin-intermediate S. aureus (VISA). - The mechanism of resistance is associated with increased cell wall synthesis and alterations in the cell wall and is not caused by the van genes found in enterococci. - S. aureus strains of intermediate susceptibility to vancomycin usually are nafcillin resistant, but generally are susceptible to oxazolidinones and to quinupristin-dalfopristin. - **Vancomycin-resistant S. aureus (VRSA):** isolated from patients in the United States. The isolates contained the vancomycin resistance gene vanA likely derived from enterococci. - **Plasmid-mediated resistance:** to tetracyclines, erythromycins, aminoglycosides, and other drugs is frequent in staphylococci. - **Tolerance:** staphylococci are inhibited by a drug, but not killed by it. ### Variation * A culture of staphylococci contains some bacteria that differ from the bulk of the population in expression of colony characteristics (colony size, pigment, hemolysis), in enzyme elaboration, in drug resistance, and in pathogenicity. * Expression of such characteristics is influenced by growth conditions. When nafcillin-resistant S. aureus is incubated at 37°C on blood agar, one in 107 organisms expresses nafcillin resistance. When it is incubated at 30°C on agar containing 2-5% sodium chloride, one in 10³ organisms expresses nafcillin resistance. * Some isolates may develop alterations in phenotypes such as smaller size (pin point colonies) and loss of hemolysis. These are referred to as small colony variants (SCVs). ### Antigenic Structure * S. aureus has amazing adaptive capacity. Full genome sequencing of numerous isolates has elucidated the evolution of various structures, toxins, and enzymes that this organism has developed over time. S. aureus has acquired many mobile genetic elements (eg, insertion sequences, transposons. etc) that determine both pathogenicity and antimicrobial resistance. * Staphylococci contain antigenic polysaccharides and proteins as well as other substances important in cell wall structure. - Peptidoglycan: a thick polysaccharide polymer containing linked subunits, provides the rigid exoskeleton of the cell wall and anchors the adhesins. - Teichoic acids: polymers of polyribitol-phosphate, are cross-linked to the peptidoglycan and can be antigenic. - Protein A: a cell wall component of S. aureus strains and is a bacterial surface protein that has been characterized among a group of adhesins called microbial surface components recognizing adhesive matrix molecules (MSCRAMMs). Bacterial attachment to host cells is mediated by MSCRAMMs, and these are important virulence factors. Protein A binds to the Fc portion of IgG molecules except IgG3. * Most S. aureus strains of clinical importance have polysaccharide capsules. - At least 11 serotypes have been identified, with types 5 and 8 responsible for the majority of infections. ### Enzymes and Toxins * Staphylococci can produce disease both through their ability to multiply and spread widely in tissues and through their production of many extracellular substances; some of these substances are enzymes; others are considered to be toxins. * The toxins are under the genetic control of plasmids; some may be under both chromosomal and extrachromosomal control; and for others, the mechanism of genetic control is not well defined. **Enzymes:** - **Catalase:** Staphylococci produce catalase, which converts hydrogen peroxide into water and oxygen. + test differentiates the staphylococci from the streptococci. - **Coagulase:** a protein that clots oxalated or citrated plasma. It binds to prothrombin. - **Clumping factor:** cell wall bound and is another example of an MSCRAMM (see earlier) that is responsible for adherence of the organisms to fibrinogen and fibrin. - **Hyaluronidase:** - **Staphylokinase:** - **Proteinases:** - **Lipases:** - **ß-Lactamase:** **Toxins:** - **Hemolysins:** - α-Hemolysin: a heterogeneous protein that acts on a broad spectrum of eukaryotic cell membranes. - β-toxin: degrades sphingomyelin and therefore is toxic for many kinds of cells, including human red blood cells. - δ-toxin: heterogeneous and dissociates into subunits in nonionic detergents. It disrupts biologic membranes and may have a role in S. aureus diarrheal diseases - γ-hemolysin: a leukocidin that lyses white blood cells and is composed of two proteins designated S and F. It can interact with the two proteins comprising the Panton-Valentine leukocidin (PVL). - **Panton-Valentine Leukocidin (PVL):** encoded on a mobile phage. It can kill white blood cells of humans and rabbits. This toxin is an important virulence factor in CA-MRSA infections. - **Exfoliative Toxins:** two distinct proteins. - Exfoliative Toxin A: encoded by eta located on a phage and is heat stable - Exfoliative Toxin B: plasmid mediated and heat labile. - **Toxic Shock Syndrome Toxin (TSST-1):** a toxin called toxic shock syndrome toxin-1 (TSST-1), which is the same as enterotoxin F. - **Enterotoxins:** heat stable and resistant to the action of gut enzymes. Important causes of food poisoning. ### Pathogenesis * Staphylococci, particularly S. epidermidis, are members of the normal microbiota of the human skin and respiratory and gastrointestinal tracts. * Nasal carriage of S. aureus occurs in 20-50% of humans. * The pathogenic capacity of a given strain of S. aureus is the combined effect of extracellular factors and toxins together with the invasive properties of the strain. * At one end of the disease spectrum is staphylococcal food poisoning, attributable solely to the ingestion of preformed enterotoxin; at the other end are staphylococcal bacteremia and disseminated abscesses in all organs. ### Clinical Findings * A localized staphylococcal infection appears as a "pimple," hair follicle infection, or abscess. * There is usually an intense, localized, painful inflammatory reaction that undergoes central suppuration and heals quickly when the pus is drained. * S. aureus infection can also result from direct contamination of a wound, such as a postoperative staphylococcal wound infection or infection after trauma. ### Diagnostic Laboratory Tests - **Specimens:** Surface swab pus or aspirate from an abscess, blood, endotracheal aspirate, expectorated sputum, or spinal fluid for culture, depending on the localization of the process, are all appropriate specimens for testing. - **Smears:** Typical staphylococci appear as Gram-positive cocci in clusters in Gram-stained smears of pus or sputum. - **Culture:** Specimens planted on blood agar plates give rise to typical colonies in 18 hours at 37°C. - **Catalase Test:** A drop of 3% hydrogen peroxide solution is placed on a slide, and a small amount of the bacterial growth is placed in the solution. The formation of bubbles (the release of oxygen) indicates a positive test result. - **Coagulase Test:** Citrated rabbit (or human) plasma diluted 1:5 is mixed with an equal volume of broth culture or growth from colonies on agar and incubated at 37°C. - **Susceptibility Testing:** Clinical laboratories adopt methods recommended by the Clinical and Laboratory Standards Institute (CLSI) or European Committee on Antimicrobial Susceptibility Testing (EUCAST). - **Serologic and Typing Tests:** Serologic tests for diagnosis of S. aureus infections have little practical value. Molecular typing techniques have been used to document the spread of epidemic disease-producing clones of S. aureus. ### Treatment - Most persons harbor staphylococci on the skin and in the nose or throat. ### Epidemiology and Control * Staphylococci are ubiquitous human pathogens. ### Pathogenesis * The prototype of a staphylococcal lesion is the furuncle or other localized abscess. * Groups of S. aureus established in a hair follicle lead to tissue necrosis (dermonecrotic factor). * Coagulase is produced and coagulates fibrin around the lesion and within the lymphatics, resulting in formation of a wall that limits the process and is reinforced by the accumulation of inflammatory cells and, later, fibrous tissue. ### Regulation of Virulence Determinants * The expression of staphylococcal virulence determinants is regulated by several systems that sense and respond to environmental signals. * The first of these systems consists of two proteins (two-component systems), an example of which is accessory gene regulator (agr).