Staphylococcus Aureus & Epidermidis

• Gram-positive cocci include staphylococci, streptococci, and enterococci.

Staphylococci General Characteristics

• Gram-positive cocci, about 1u in diameter, arranged in grape-like clusters.
• Non-capsulated, non-motile, and non-spore forming.

Classification of Staphylococci

• Staph. aureus is the most important pathogen and is coagulase positive.
• Staph. epidermidis and Staph. saprophyticus are coagulase negative and are normal human flora, but can sometimes cause human infection.

Staphylococcus aureus: Morphology

• Gram-positive cocci are arranged in irregular, grape-like clusters.
• Non-motile and non-sporing.

Staphylococcus aureus: Culture

• Facultative anaerobes.
• Grows on most bacteriologic media at 37°C.
• On nutrient agar, it produces golden yellow colonies.
• On blood agar, Staph. aureus produces complete hemolysis.
• Mannitol salt agar is a selective medium containing 7.5% NaCl for isolating Staph. aureus from contaminated samples.
• Staph. aureus gives a yellow color on mannitol salt agar due to mannitol fermentation.

Biochemical Reactions of Staphylococcus aureus

• Catalase test: Positive
• Coagulase test: Positive
• DNAase test: Positive
• Ferments mannitol.
• Causes gelatin liquefaction.

Pathogenesis, Antigenic Structure, and Virulence Factors of Staphylococcus aureus

• Involves invasion via antigenic structures and extracellular enzymes in the body.
• Antigenic structures include peptidoglycan, teichoic acids, protein A, and surface receptors.
• Extracellular enzymes include catalase, coagulase, clumping factor, and others.
• Produces toxins such as toxic shock syndrome toxin (TSST-1), enterotoxins, and 3-Exfoliative toxins.

Antigenic Structure of Staph. aureus

• Several important cell wall antigens are present.
• Peptidoglycan is a polysaccharide polymer that elicits production of interleukin-1 (endogenous pyrogen).
• Acts as a chemoattractant for polymorphonuclear leukocytes and has endotoxin-like activity.
• Teichoic acids are polymers of ribitol phosphate linked to peptidoglycan, facilitating adherence of the staphylococci to mucosal cells.
• Protein A binds to the Fc portion of IgG molecules, freeing the Fab portion to combine with a specific antigen, preventing complement activation.
• Surface receptors for specific bacteriophages permit "Phage typing" for epidemiologic purposes.

Enzymes Produced by Staphylococcus aureus

• Catalase converts hydrogen peroxide into H₂O and O₂ and differentiates staphylococci (positive) from streptococci (negative).
• Coagulase clots citrated plasma by converting fibrinogen into fibrin, causing staphylococci to become more resistant to phagocytosis, resulting in localized infections.
• Clumping factor is on the Staph. aureus cell wall surface and binds to fibrinogen, yielding bacterial aggregation and attachment to blood clots, and this is distinct from coagulase.
• Other enzymes include hyaluronidase, staphylokinase, proteinases, lipases, and β-lactamase

Toxins Produced by Staphylococcus aureus

• Toxic shock syndrome toxin (TSST – 1) causes shock in tampon-using menstruating women, individuals with wound infections, and patients with nasal packing.
• TSST-1 is produced locally in the vagina, nose, or other infected site, entering the bloodstream and causing toxemia, the toxin is a superantigen that stimulates the release of IL-1, IL-2, and tumor necrosis factor (TNF).
• Enterotoxins act as superantigens, causing food poisoning with vomiting and watery diarrhea by stimulating the enteric nervous system.
• It is heat-resistant, not inactivated by brief cooking, resistant to stomach acid and enzymes, and has many types (A-E, G-I, K-M).
• Exfoliative toxins are proteins A and B that yield desquamation seen in staphylococcal scalded skin syndrome (peeling of the skin) and they are superantigens.
• Leukocidins kill leukocytes resulting in tissue necrosis, Panton-Valentin leukocidin (PVL) produced by community-associated CA-MRSA, is associated with severe necrotizing pneumonia in children.

Diseases Caused by Staph. Aureus

• Pyogenic diseases result in localized superficial infections of carbuncles, boils, impetigo, wound infection, and abscess formation.
• Deeply seated infections occur if Staph. aureus disseminates and causes bacteremia, leading to osteomyelitis, meningitis, pneumonia, urinary tract infections, and endocarditis.
• Toxigenic diseases include food poisoning.
• Food poisoning results from ingesting enterotoxin in improperly cooked food that isn’t refrigerated.
• Food poisoning has a short incubation period (1-8 hours).
• The source of infection is often the nose and hands of food handlers or the food handlers themselves.
• Carbohydrate-rich foods like cakes, milk, and pastries are often the source.
• Symptoms include nausea, vomiting, and diarrhea, without fever.
• Toxic shock syndrome, in response to TSST-1, manifests with an abrupt onset of high fever, vomiting, diarrhea, myalgia, macular and sunburn-like rash, hypotension, and cardiac/renal failure.
• Scalded skin syndrome (SSS) is a response to exfoliative toxin, in which the superficial layer of the epidermis sloughs off.

Laboratory Diagnosis of Staphylococcus aureus Infections

• Specimens are collected according to the site of infection.
• Blood is taken for blood cultures in septicemia, bacteremia, and endocarditis.
• Swabs are taken from lesions (pus) in superficial pyogenic infections.
• CSF is collected in cases of meningitis. Vomitus, stool, or food remnants are collected in food poisoning cases.
• Joint aspirates are taken in arthritis cases.
• Sputum in is collected in cases of Pneumonia.
• Gram-stained smears show Gram-positive cocci arranged in grape-like clusters among pus cells.
• Culture takes place at 37 degrees for 24 hours.
• Nutrient agar, blood agar, and mannitol salt agar are used.
• Gram-stained films show Gram-positive cocci arranged in grape-like clusters.
• Catalase tests, coagulase tests, and DNAase tests are all positive.
• Phage typing is used to determine the source of infection.
• Performed for epidemiological tracing of outbreaks, testing samples from wounds, nose, nail beds of medical staff, and fomites.
• In food poisoning outbreaks, isolates come from food, vomitus, stool, and nose/nail beds of food handlers.
• Serological identification can be done via latex agglutination test.
• Molecular identification is performed by PCR or DNA probes.

Antibiotic Sensitivity Testing

• Used to determine which antibiotics bacteria are susceptible to.

Diagnosis of Staphylococcal Food Poisoning

• Specimen can be vomitus, stools, or food remnants.
• Perform isolation and identification as above using mannitol salt agar.
• Demonstration of enterotoxin is done by precipitation (gel diffusion) method.
• Bacteriophage typing is used to trace the source of infection in outbreaks.

Treatment of Staphylococcus aureus Infections

• 90% of S. aureus strains produce β-lactamase due to transmissible plasmids.
• Often susceptible to β-lactamase-resistant penicillins like methicillin or oxacillin, cephalosporins, or vancomycin.
• 20% of S. aureus strains are "methicillin"-resistant (MRSA).
• Vancomycin is the drug of choice for these staphylococci.
• Newer antimicrobial agents such as linezolid and daptomycin are also available.

Prevention of Staphylococcus aureus Infections

• Cleanliness and frequent handwashing are key.
• Aseptic management of lesions can help control spread of Staph aureus.
• Persistent nasal colonization of Staph aureus can be reduced (but not completely eliminated) by intranasal mupirocin.
• Shedders may have to be removed from high-risk areas.
• High-risk areas are operating rooms and newborn nurseries.

Diseases Caused by Coagulase-Negative Staph

• Staph. epidermidis is an opportunistic pathogen and a part of the normal human flora on the skin and mucous membranes.
• Predisposing factors for Staph. epidermidis infections are the bloodstream via intravenous catheter.
• Prosthetic implants can introduce Staph. epidermidis (prosthetic heart valves, vascular grafts, and prosthetic joints).
• Infection caused by the production of glycocalyx, and can cause sepsis in neonates, and peritonitis in patients undergoing peritoneal dialysis
• Staph. saprophyticus causes urinary tract infections, particularly in sexually active young women.
• Is second to Escherichia coli as a cause of community-acquired urinary tract infections in young women

Key Differences Between

• S. epidermidis* and S. saprophyticus:
• Colonial morphology: S. epidermidis forms white, non-hemolytic colonies while S. saprophyticus forms variable colored non-hemolytic colonies.
• Mannitol fermentation: Both S. epidermidis and S. saprophyticus tests are negative.
• Dnase test: Both S. epidermidis and S. saprophyticus tests are negative.
• Novobiocin sensitivity: S. epidermidis is sensitive, while S. saprophyticus is resistant.

Important Staphylococcus Species

• Staph. aureus is coagulase +ve, exhibits beta hemolysis, produces protein A on the cell surface, and causes suppurative lesions.
• Staph. epidermidis is coagulase -ve, exhibits no hemolysis, is sensitive to novobiocin, and is a common member of skin flora.
• Staph. saprophyticus is coagulase -ve, exhibits no hemolysis, is resistant to novobiocin, sometimes causes urinary tract infections, and is normal flora of skin.

Test your knowledge of Staphylococcus aureus and epidermidis. Questions cover characteristics, pathogenesis, clinical ID, and treatment. Quizzes focus on the role of teichoic acids and peptidoglycans in bacterial infections.