Unit 1 - Staph, Strep, Enterococci

What is the Gram stain morphology of Staphylococcaceae?

Gram-positive cocci

What are the four arrangements of Gram-positive cocci mentioned?

Pairs, clusters, tetrads, chains

How do old bacteria appear on a Gram stain?

They stain lighter shades of blue-purple to pink (do not retain crystal violet)

What is the atmospheric requirement for most Staphylococcaceae?

Facultative anaerobe

What type of incubator do most clinical labs use for aerobic identification of Staphylococcaceae?

3-5% CO₂ incubator

What is the mechanism of action of beta-lactam antibiotics?

They interfere with cell wall synthesis (peptidoglycan production)

What is the first-choice antimicrobial therapy for Staphylococcus?

Penicillins, oxacillin, ampicillin cephems

What is the Canadian Nosocomial Infection Surveillance Program (CNISP)?

A program by the Public Health Agency of Canada that tracks rates and trends of healthcare-associated infections.

What does CNISP provide to help develop national guidelines?

Data on clinical issues related to healthcare-associated infections.

What was the former family name for Staphylococcus?

Micrococcaceae

What is the cellular arrangement of Staphylococcus sp.?

Gram-positive cocci in grape-like clusters

What is the colony morphology of most Staphylococcus strains after 18-24 hours of growth?

Smooth, butyrous (soft, warm butter), cream-colored/white

Are most strains of S. aureus beta-hemolytic?

Yes

What is the primary reservoir for Staphylococcus sp. in the human body?

Nares (nose)

What percentage of the population carries Staphylococcus sp. as normal flora?

20-40%

What are the secondary sites of Staphylococcus sp. colonization?

Axillae, vagina, pharynx, skin

Are MRSA and VRSA more common in healthcare professionals?

Yes, higher rates are found in hospital personnel.

What are the two main categories of Staphylococcus infections?

Suppurative (invasive) and toxin-mediated

What are the invasive skin infections caused by Staphylococcus aureus?

Boils (furuncle, carbuncle), folliculitis, cellulitis

What are the invasive organ infections caused by Staphylococcus aureus?

Pneumonia, endocarditis, osteomyelitis, septicemia

What is the causative agent of Staphylococcal food poisoning?

Ingestion of a heat-stable enterotoxin (A-E) produced by S. aureus

How long after ingestion of contaminated food do symptoms of Staphylococcal food poisoning appear?

2-8 hours

What foods are commonly associated with Staphylococcal food poisoning?

Meat or dairy products

What is the toxin responsible for Staphylococcal Scalded Skin Syndrome?

Pyogenic exfoliating toxin (exfoliative toxin)

What are the symptoms of Toxic Shock Syndrome (TSS)?

Fever, chills, vomiting, diarrhea, aches, rash, hypotension, shock, multi-system organ failure

What is the mortality rate of Toxic Shock Syndrome?

2-5%

What is the causative agent of Toxic Shock Syndrome?

Toxic shock syndrome toxin-1 (TSST-1) from S. aureus

Which CoNS species is a normal skin flora and a common cause of nosocomial infections?

Staphylococcus epidermidis

What is the main virulence factor of Staphylococcus epidermidis that allows it to infect prosthetic devices?

Biofilm production

What is the second most common cause of female cystitis (ages 16-30)?

Staphylococcus saprophyticus

What type of infection is Staphylococcus lugdunensis commonly associated with?

Endocarditis, osteomyelitis, septicemia

What is the normal habitat of Staphylococcus lugdunensis?

Commensal (normal flora, mixed indigenous flora)

What are the predisposing factors for Staphylococcus infections?

Defects in leukocyte chemotaxis, skin injuries, foreign bodies, viral infections, chronic diseases, antibiotic treatment

What are Staphylococcal enterotoxins?

They are heat-stable exotoxins that cause food poisoning.

What is Toxic shock syndrome toxin-1 (TSST-1)?

It is a superantigen that elicits an aggressive immune response leading to multi-system failure.

What does exfoliative toxin do?

It causes the skin to slough off (Scalded Skin Syndrome).

What do cytolytic toxins do (alpha, beta)?

They affect red blood cells and white blood cells (seen as hemolysis on SBAP).

What enzyme do all Staphylococcus species produce?

Catalase

What enzyme is specific to Staphylococcus aureus?

Coagulase

What is the function of hyaluronidase in Staphylococcus?

It hydrolyzes hyaluronic acid, aiding in the spread of infection.

What is the function of Protein A in Staphylococcus aureus?

It binds to the Fc portion of IgG to block phagocytosis.

What is the function of the Staphylococcus capsule?

It resists phagocytosis.

What is Extracellular Slime Substance (ESS) and what does it do?

A substance produced by all Staphylococcus sp. that contributes to biofilm formation, especially on plastic.

What is the purpose of the Catalase test?

To differentiate Staphylococcus (positive) from Streptococcus (negative).

What is the reagent used in the Catalase test?

3% hydrogen peroxide (H₂O₂)

What is a positive result for the Catalase test?

Immediate bubbling (production of oxygen)

What are sources of error for the Catalase test?

Incubation >100 min (false +), picking up RBCs (false +), culture older than 24 hrs (false -), selective media (false +/-)

What is the purpose of the Coagulase test?

To differentiate Staphylococcus aureus (positive) from other Staphylococcus species (negative).

What is the reagent used in the slide coagulase test?

EDTA rabbit plasma

What does the slide coagulase test detect?

Clumping factor (bound coagulase)

What is a positive result for the slide coagulase test?

Clumping in <10 seconds

What is the purpose of the tube coagulase test?

To detect free coagulase and confirm negative slide coagulase results.

What does the tube coagulase test detect?

Free coagulase (staphylocoagulase) and bound clumping factor.

What is a positive result for the tube coagulase test?

Any type of clot formation (fibrin clot)

How often should the tube coagulase test be observed?

Every 30 minutes up to 4 hours, then up to 24 hours overnight.

What are sources of error for the coagulase test?

High salt media (false +), mixing too long (false +), contaminated plasma (false +), citrated plasma (false +/-)

What is the "Drop, drop, rock method" used for?

Rapid latex agglutination tests for S. aureus identification.

What does the latex agglutination test for S. aureus detect?

Protein A and clumping factor

What is the purpose of the DNase test?

To identify S. aureus (DNase positive).

What is the substrate in the DNase test?

DNA

What is a positive result for the DNase test?

A zone of clearing around the colony growth

What is the purpose of the Novobiocin susceptibility test?

To differentiate Staphylococcus saprophyticus (resistant) from other CoNS.

What is the Novobiocin resistance breakpoint for S. saprophyticus?

What is the Novobiocin susceptibility breakpoint for other CoNS?

≥16 mm (Susceptible)

What does a positive Modified Oxidase test indicate?

Micrococcus (positive) vs Staphylococcus (negative)

What is the result of the Bacitracin test for Staphylococcus?

Resistant (growth up to the edge of the disc)

What is the result of the Bacitracin test for Micrococcus?

Susceptible (any zone of inhibition)

What is the PYR result for Staphylococcus aureus?

Negative

What is the ODC result for Staphylococcus aureus?

Negative

What is the colony morphology of Staphylococcus epidermidis?

1 mm white, non-hemolytic

What is the mannitol fermentation result for Staphylococcus saprophyticus?

Positive (late fermenter)

What is the novobiocin result for Staphylococcus saprophyticus?

Resistant

What is the coagulase result for Staphylococcus lugdunensis?

Slide coagulase positive, tube coagulase negative

What is the ODC result for Staphylococcus lugdunensis?

Positive

What is the PYR result for Staphylococcus lugdunensis?

Positive

What is the colony morphology of Staphylococcus lugdunensis?

Small to medium (1-2 mm), sticky, yellow or tan, usually beta hemolytic

What is the mecA gene?

The gene encoding for methicillin resistance in staphylococci.

What is the product of the mecA gene?

Penicillin-binding protein 2a (PBP2a)

Why is oxacillin/cefoxitin resistance clinically significant?

It indicates resistance to all beta-lactam antimicrobials (penicillins, cephalosporins, carbapenems).

What is the CLSI recommendation for detecting oxacillin resistance in S. aureus?

Use a 30 µg cefoxitin disc as a surrogate.

What is the incubation time for cefoxitin disc diffusion for S. aureus?

16-20 hours

What is the incubation time for cefoxitin disc diffusion for coagulase-negative Staphylococcus?

24 hours

What should be reported instead of cefoxitin when testing for oxacillin resistance?

Report oxacillin or methicillin, never cefoxitin.

What is the purpose of adding 2% NaCl to Mueller Hinton agar when testing for MRSA?

To improve detection of heteroresistant strains.

How is oxacillin resistance interpreted on disc diffusion?

Any discernible growth within the zone is considered resistance.

What is the Oxacillin Salt Agar Screening test?

A screening method for MRSA carriers using Mueller Hinton agar with 4% NaCl and 6 µg/mL oxacillin.

What is the incubation time for Oxacillin Salt Agar Screening?

24 hours (up to 48 hours)

What does any growth on Oxacillin Salt Agar Screening indicate?

Resistance (MRSA)

What is the MRSA screen slide agglutination test detecting?

Penicillin-binding protein 2' (PBP2a)

What is the purpose of Beta Lactamase testing?

To predict resistance to penicillin, ampicillin, and other penicillins.

What is the method of choice for Beta Lactamase testing?

Nitrocefin chromogenic test

What is a positive result for the Nitrocefin test?

Red color

Why must Staphylococcus be exposed to a penicillin-type antibiotic before Beta Lactamase testing?

The enzyme is inducible.

What is the result if Beta Lactamase is positive?

All penicillins are reported as Resistant (R).

What is the result if Beta Lactamase is negative?

Penicillins are reported based on susceptibility test results (S, I, R).

What is the historical significance of Vancomycin?

It was the last-chance treatment for Gram-positive organisms.

How is Vancomycin resistance detected?

Vancomycin agar screening or MIC methods (disc diffusion is not reliable).

What is the purpose of the D-zone test?

To detect inducible clindamycin resistance in Staphylococcus.

What is the procedure for the D-zone test?

Place clindamycin and erythromycin discs 15-20 mm apart.

What is a positive D-zone test result?

Flattening of the clindamycin zone nearest the erythromycin disk.