Module 9 Flashcards by Taylor Horlock

How many Staph species are there?

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When is lab identification to the species level required?

For common human pathogens.

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What are the two groups of Staph species and organisms that belong to them?

Coagulase positive- S. aureus, S. schleiferi, S. hyicus, S. intermedius

Coagulase negative- S. epidermidis, S. saprophyticus, S. lugdunensis

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What is the cellular morphology of Staph?

Gram positive cocci

Spherical

0.4-1.2um

Usually clusters

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What are the growth requirements of Staph?

Facultative anaerobes

Optimum temperature- 35-37°C

Easy to grow, medium can be enriched

No growth on MacConkey

Nonmotile

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What is the colonial morphology of Staph?

Rapid growth on BAP

Opaque, dense

Varying degrees of B-hemolysis

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What are the different types of B-hemolysis?

B- complete RBC clearing

a- partial RBC clearing

gamma- no RBC clearing

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What is the colonial morphology of S. aureus?

Creamy-yellow colonies (lipochrome pigment after 48hrs)

Usually narrow zone of B-hemolysis, some strains show double zone and others are nonhemolytic

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What is the colonial morphology of CNS?

Usually white

Smaller than S. aureus

Usually gamma hemolytic

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What media is used for Staph selection and why?

Mannitol salt agar- 7.5% NaCl

Allows staph growth while inhibiting others

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How does MSA differentiate between different Staph species?

Fermentation of mannitol produces acid that is detected by phenol red.

Growth- Staph

Mannitol negative (red)- CNS

Mannitol positive (yellow)- S. aureus

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What is MSA used for?

Screening healthcare workers for S. aureus.

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What is the genus identification of Staph?

Catalase positive

Nitrate positive (after reagent addition)

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What is the species identification for S. aureus?

Slide coagulase test, detects bound coagulase/clumping factor produced.

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What is done if a suspected S. aureus species gives a negative slide coagulase test?

Tube coagulase performed.

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Why are the two other Staph species that give positive slide coagulase results a nonissue?

They are animal isolates.

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What does hemagglutination test for?

Bound coagulase

Results in immediate clumping of RBCs sensitized with fibrinogen.

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How are latex reagents used to test for S. aureus identification?

Latex particles are attached to fibrinogen and IgG.

Reacts with the Protein A antibody in S. aureus to produce agglutination.

If protein A (S. aureus) is present, the IgG and latex will be bound.

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What is the purpose of the DNase test?

Confirmation of S. aureus identification.

Occasional negative test.

4% CNS are positive.

What is the purpose of the thermos table nuclease test?

Definitive test for S. aureus, only organism that produces the enzyme.

Food microbiology, not routinely performed.

When does S. saprophyticus need to be identified down to the species level?

When isolated from urine in significant amounts.

What is the colonial morphology of S. saprophyticus?

1-2mm on BAP, gamma hemolysis, bright white colonies that run together

No growth of MacConkey

Nonmotile

What results does S. saprophyticus give on novobiocin susceptibility tests?

Resistant zones

Could also be Micrococcus

What are the results of a bacitracin test?

Staph- resistant

Micrococcus- susceptible

What are the characteristics of Micrococcus?

Normal skin flora Grows on BAP- small, nonhemolytic, many colours Catalase positive Gram positive

What is S. aureus the most common cause of?

Skin, soft tissue and post traumatic infections. Second most acquired hospital infection.

What virulence factors does S. aureus produce?

Leucocidin Hyaluronidase Staphylokinase Hemolysins DNase Coagulase B-lactamase

What common infections are cause by S. aureus?

Furuncle (boil) Carbuncle Stye Impetigo Abscesses (deep soft tissue) Joint infections Infected cuts, abrasions, trauma wounds, incisions Pneumonia Osteomyelitis Septicaemia

What diseases are caused by S. aureus exotoxins?

Food poisoning- heat stable enterotoxin TSS- grows in tampon Scalded skin syndrome- mostly children and immunocompromised patients

What is the pathogenicity of S. saprophyticus?

Primary urinary pathogen. Mostly young women and older men.

What is the pathogenicity of other CNS (not S. saprophyticus)?

Normal flora found on skin, nares and genital mucosa. Opportunistic, leads to generalized infections from shunts, prosthetic heart valves and joints, IV catheters, dialysis procedures. Septicaemia Meningitis UTIs

What is the antimicrobial susceptibility of S. aureus?

Resistance for penicillin G has increased. Due to B-lactamase.

How is B-lactamase detected in S. aureus?

0.5 MF prepared Inoculated with low concentrations of oxacillin or methicillin. Growth from edge of zone of inhibition is rubbed on the B-lactamase disc/strip Resistance can also be determined by antimicrobial susceptibility testing.

What antimicrobials is MRSA resistant to?

All penicillins and cephalosporins. Vancomycin is often the only choice in treatment.

What makes identifying MRSA difficult?

Slow growing Small colonies that can resemble CNS May give negative slide coagulase test (requires full 24hrs) Should perform tube coagulase with in 24hrs

What are heteroresistant strains?

Those which contain two subpopulations. S. aureus* has one sensitive to methicillin and one resistant.

How is S. aureus detected if it displays heteroresistance?

Resistant population grows slower therefore prepared directly. Incubated at 30-35°C. 2% NaCl to enhance growth. Oxacillin discs give small zone of inhibition, measure inside irregular growth.

Can microdilution and Vitek techniques detect MRSA?

No but they may flag possible specimens.

What is the screen test for MRSA?

MH agar with oxacillin and 4% NaCl. Spot inoculate and incubate. Any growth indicates resistance.

What new MRSA drugs are there?

Linezolid Quinupristin Dalfopristin Daptomycin Tigecydine

What is the antimicrobial susceptibility of CNS species?

Generally more resistant. Mostly normal flora.

What is S. saprophyticus normally treated with and why?

Less resistant than other CNS. Treated with quinolones (UTI).

What is epidemiology? Give an example.

The study of the sources and spread of infection. Testing hospital workers for S. aureus after an outbreak.

What are methods of fingerprinting MRSA isolates?

Bacteriophage typing- phages lyse host bacteria, very specific, ID based on lysis patterns Pulse field gel electrophoresis- cleave chromosome sections and separate on a gel by an electrical current Mec A gene testing- PCR for gene that codes for resistance to methicillin and other B-lactam antimicrobials

What family does the Staph genus belong to?

Micrococcaceae