Module 9: Staphylococcus Flashcards
What other organism is the the Micrococcaceae family that may be confused with Staphylococcus?
Micrococcus
How are the different species of staphy separated?
Coagulase positive and coagulase negative
What is the most common species of CNS?
S. epidermidis
What are 2 other common species of CNS? (Other than S.e)
S. saprophyticus and S. lugdunensis
What are the growth requirements for staph?
Facultative anaerobes, larger colonies in ambient air
35-37 degrees
Grow on most media
Negative motility (atrichous)
What are the cultural characteristics/colony morphology of staph?
1-4mm (rapid growing, S. aureus usually bigger)
Opaque, dense
White or yellow (yellow usually S. aureus)
Beta or gamma hem (beta usually S. aureus, may be double)
What causes S. aureus to be yellow in color on BA?
Lipochrome (after 48h incubtion)
May be enhanced by longer incubation and lower temperature after colonies have formed
What is the selective media used for staph? What makes it selective? Is it differential?
Mannitol salt agar
7.5% sodium chloride (staph can withstand high salt concentration)
It is differential by having mannitol and phenol red
How is staph differentiated on MSA?
Mannitol fermenters produce acid and turn yellow
Red colonies - probably CNS
Yellow colonies - probably S. aureus
What Staphylococcus is capable of fermenting mannitol?
S. aureus
MSA is not commonly used. When might it be included?
Screening health care workers for S. aureus
What are the genus identification test for staph?
Catalase - positive
Nitrate - reduces nitrate to nitrite (red after reagents a and b)
What are the species identification tests for staph?
Coagulase, slide and tube Hemagglutination Latex Reagents DNase Thermostable Nuclease
How does the hemagglutination test work?
Commercial reagent consisting of RBCs sensitized with fibrinogen
Bound coagulase causes clumping
Positive = S. aureus
How does the latex reagent test work?
Commercial latex reagent has latex particles with fibrinogen and IgG antibodies
Bound coagulase and protein A cause clumping
Positive = S. aureus
What is protein A? What staph has it?
Cell wall protein in S. aureus
Binds to nonspecific IgG and Fc portion
How doe the thermostable nuclease test work?
S. aureus produces a thermostable nuclease enzyme
Boiled and tested for the enzyme
Not really used in lab, useful for food micro (food poisoning)
What are the results for all the possible genus and species tests for CNS?
Catalase - positive Nitrate - reduces nitrate to nitrite Coagulase (slide and tube) - negative Hemagglutination - negative Latex agglutination - negative DNase - negative Thermostable nuclease - negative Protein A - negative
What are the results for all the possible genus and species tests for S. aureus?
Catalase - positive Nitrate - reduces nitrate to nitrite Coagulase (slide and tube) - positive Hemagglutination - positive Latex agglutination - positive DNase - positive Thermostable nuclease - positive Protein A - positive
What is the colonial morphology of S. saprophyticus?
1-2mm, gamma hemolysis, white, round, opaque
Often appear wet/glistening, may run together
How can S. saprophyticus be differentiated from other CNS?
When isolated in significant amount from the urinary tract
Differentiated by a Novobiocin disc
ZOI = 12mm = resistant, S. saprophyticus
ZOI > 12mm = sensitive, other CNS
How can staph be differentiated from Micrococcus? Why is this done?
Differentiated by a Bacitracin disc
resistant = staph
susceptible = Micrococcus
Done if ID is required on CNS
How might the species of a CNS be determined?
Biochemical tests Commercial systems (strips, trays, with biochemicals on/in them, generates number profile and bug ID'd with database) Automated Instruments (Vitek, MALDI)
What is Mircrococcus?
Normal flora of skin, mucous membranes, and oropharynx
Grows on BA as small, non-hem, various colors
Catalase positive
Rarely cause infection
What virulence factors does S. aureus have?
Leucocidin - kills WBCs
Hyaluronidase - breaks down hyaluronic acid of tissue, allows spreading
Staphylokinase - lyses clots, allows spreading
Hemolysins - lyses RBCs
Others: DNase, coagulase, beta-lactamase
What are some common S. aureus infections?
Furuncle (boil), carbuncle, stye, impetigo, abscesses, joint infections, infected cuts/abrasions/wounds, infected incisions, pneumonia, osteomyelitis, septicemia
What is a furuncle?
Localized abscess around hair follicle
What is a carbuncle?
Several boils
What is a stye?
Infection around an eyelash
What is impetigo?
Skin infection, watery blisters
What is an abscess?
Deep soft tissue infection
What can S. aureus’s exotoxins cause?
Food poisoning, toxic shock syndrome, scalded skin syndrome
How does S. aureus cause food poisoning?
It has a heat stable enterotoxin (type of exotoxin)
Occurs in 1-6 hours, vomiting and diarrhea
How does S. aureus cause toxic shock syndrome?
Grows in a tampon and liberates toxin when absorbed and spread by the blood
How does S. aureus cause scalded skin syndrome?
Usually in immunocompromised and children
Boil or skin infection, toxin leads to sunburn-like rash and desquamation leaving red dermis exposed
How pathogenic is S. sapropyticus?
Primary urinary pathogen, especially in young women
How pathogenic is CNS?
Usually normal flora (skin, mucous membranes (nose, genitals) Opportunistic pathogen (immunocompromised, shunts, prosthetic heart valves and joints, dialysis, surgery, etc)
What is the usual antimicrobial susceptibility of S. aureus?
85-90% resistant to penicillin G (beta-lactamase)
What is MRSA?
Methicillin Resistant S. aureus (20-30% of S. aureus)
Important nosocomial pathogen
Resistant to all penicillin and cephalosporins, and most other antibiotics
Usually must be treated with vancomycin
Why is it difficult to identify MRSA?
Some are slow growing, and produce small colonies that resemble CNS and may give negative slide coag results
What might be caused if a CNS infection becomes generalized?
Septicemia and meningitis
What antibiotic is S. aureus now resistant t that it used t be universally susceptible to? Why?
Benzylpenicillin (Pen G)
Due to beta-lactamase enzyme, the genetic information is carried on my plasmids
What happens if S. aureus is not positive for beta-lactamase on the primary plate?
It must be induced by growing S. aureus in the presence of low concentrations of penicillin or other beta-lactam antibiotic, then repeating the test
What is a heteroresistant strain of MRSA?
S. aureus with 2 subpopulations, 1 resistant to methicillin, one susceptible
What is the standard inoculum for MRSA testing?
0.5 MacFarland standard of colonies from the primary plate, incubated at 30-35 degrees for a full 24 hours
2% NaCl may be added to the broth for MIC tests to encourage the resistant strain to grow
What is the disc diffusion method for MRSA testing?
Oxacillin disc used
MRSA grows in towards the disc and appears feathered, whereas MSSA would have a clear distinct ZOI
What is the microdilution MIC method for MRSA testing?
Vitek system, inadequate for detecting MRSA, possibly due to small inoculum and short incubation
May “flag” possible MRSA based on resistance patterns
What is the MRSA oxacillin screen test?
Mueller Hinton agar with 6ug of oxacillin/mL and 4% sodium chloride
Standard inoculum struck on plate, incubated to 35 for 24 hours
ANY growth = resistance
Example of agar dilution method (antimicrobial is in medium)
What other drugs is MRSA usually resistant to?
Erythromycin, clindamycin, chloramphenicol, ciprofloxacin, and aminoglycosides
Cephalosporins may be sensitive, but are ineffective
What is the susceptibility of CNS?
Usually more resistant than S. aureus, but unpredictable
Testing done each time
What is the susceptibility of S. saprophyticus?
Least resistant staph, susceptible to most antimicrobials
Usually quinolones used: norfloxacin, nitrofurantoin
Define epidemiology
Study of the sources and spread of infection
What is the most common place for S. aureus to be carried?
Nares (30% of adults, 50% or more or hospital workers)
Where might S. aureus be carried?
Mucous membranes, skin, and the intestine
What may be done to trace the epidemiology of MRSA?
Bacteriophage typing by pulse-field gel electrophoresis
What is a bacteriophage?
A virus that infects bacterial cells
How does bacteriophage typing work?
Phages lyse host bacteria resulting in plaques or clear areas in bacteria growth because phages are host specific
What is pulse field gel electrophoresis? How does it work?
DNA technology which cleaves sections of the bacteria chromosome in smaller pieces which are separated on gel by a pulsed electrical current
Gives DNA fingerprint of the organism
What is the Mec A gene?
Mec A gene codes for resistant to methicillin and other beta-lactamase antibiotics
The species of coagulase negative Staphylococcus most commonly isolated from clinical material is
a. aureus
b. intermedius
c. saprophyticus
d. epidermidis
d. epidermidis
Which one of the following is not a morphological characteristics of the genus Staphylococcus
a. grape like clusters
b. one micrometer in dimension
c. oval shaped cells
d. short chains of cells
c. oval shaped cells
The same culture of S. aureus subcultured to aerobic and anaerobic blood agar would yield
a. growth only on the aerobic plate
b. growth only on the anaerobic plate
c. larger colonies on the anaerobic plate than the aerobic plate
d. larger colonies on the aerobic plate than on the anaerobic plate
d. larger colonies on the aerobic plate than the anaerobic plate
A typical strain of S. epidermidis grown overnight on blood agar would appear
a. opaque, white, non-hemolytic
b. translucent, gray, non-hemolytic
c. creamy, opaque, beta hemolytic
d. yellow, opaque, non-hemolytic
a. opaque, white, non-hemolytic
Which of the following would enhance typical S. aureus pigmentation
a. incubating 18 hours in ambient air at 35 degrees
b. incubating 18 hours anaerobically at 35 degrees
c. incubating 18 hours in ambient air at 35 degrees and a further 18 hours at room temperature
d. incubating in broth for 48 hours at 22 degrees
c. incubating 18 hours in ambient air at 35 degrees and a further 18 hours at room temperature
The selective ingredient in mannitol salt agar is
a. mannitol
b. salt
c. agar
d. phenol red
b. salt
On mannitol salt agar, a typical strain of CNS
a. does not grow
b. grows and produces yellow zones around colonies
c. grows with a red color around the colonies
d. grows with clear zones around the colonies
c. grows with a red
Phenol red used in mannitol salt agar
a. is red when acid
b. is yellow when acid
b. is yellow when acid
Choose which organism matches the statement
a. S. aureus
b. CNS
c. both
d. neither
Reduces nitrate to nitrite
c. both
Choose which organism matches the statement
a. S. aureus
b. CNS
c. both
d. neither
Ferments mannitol
a. S. aureus
Choose which organism matches the statement
a. S. aureus
b. CNS
c. both
d. neither
Has protein A in its cell wall
a. S. aureus
Choose which organism matches the statement
a. S. aureus
b. CNS
c. both
d. neither
Is DNase positive
a. S. aureus
Choose which organism matches the statement
a. S. aureus
b. CNS
c. both
d. neither
Is tube coagulase positive
a. S. aureus
Choose which organism matches the statement
a. S. aureus
b. CNS
c. both
d. neither
Is slide coagulase positive
a. S. aureus
Choose which organism matches the statement
a. S. aureus
b. CNS
c. both
d. neither
Is catalase positive
c. both
Choose which organism matches the statement
a. S. aureus
b. CNS
c. both
d. neither
Has a thermostable nuclease protein
a. S. aureus
A hospital is checking nursing staff for S. aureus carries by culturing nasal swabs on mannitol salt agar . Which colonies should be subcultured to blood agar for investigation as possible S. aurues?
Select colonies that have a yellow color surrounding them
A stock culture of S. aureus failed to give a positive result for the catalase test. What is the most likely reason for this?
The reagent is likely too old and has broken down into water
The most reliable single test for identification of S. aureus is
a. slide coagulase
b. tube coagulase
c. mannitol fermentation
d. DNase
b. tube coagulase
Protein A
a. is a cell wall protein
b. is specific for S. aureus
c. combines with the Fc part of IgG
d. all of the above
d. all of the above
is a cell wall protein, is specific for S. aureus, and combines with the Fc part of IgG
Protein A may be detected by reaction with
a. sheep red blood cells
b. fibrinogen
c. rabbit plasma
d. IgG attached to latex particles
d. IgG attached to latex particles
A test useful for detecting S. aureus directly from filtrates of food is
a. thermostable nuclease
b. deoxyribonuclease
c. beta-lactamase
d. coagulase
a. thermostable nuclease
Which of the following tests will differentiate the genus Staphylococcus from Micrococcus
a. resistance to novobiocin
b. resistance to bacitracin
c. resistance to benzylpenicillin
d. resistance to methicillin
b. resistance to bacitracin
Most coagulase negative staphylococci can be identified to the species level
a. the Vitek auto microbic automated system
b. a series of conventional biochemical tests
c. a micro test strip such as API
d. all of the above
b. a series of conventional biochemical tests
Coagulase negative staphylococci are the predominant normal flora organisms of the
a. upper respiratory tract
b. intestinal mucosa
c. genital mucosa
d. skin
d. skin
Predisposing factors that may lead to an infection with coagulase negative staphylococci include all of the following except
a. artificial heart valve
b. cerebral spinal fluid shunt
c. indwelling intravascular catheter
d. laceration of the epidermis
d. laceration of the epidermis
A coagulase negative Staphylococcus may be clinically significant when isolated from
a. nares
b. blood
c. hair
d. ears
b. blood
Which of the following is not an invasive factor common to S. aurues
a. streptokinase
b. hyaluronidase
c. leukocidin
d. hemolysin
a. streptokinase
Which of the following infections is not commonly caused by S. aureus
a. impetigo
b. furuncle
c. pharyngitis
d. septicemia
c. pharyngitis
Which of the following diseases is not primarily caused by an exotoxin of S. aureus
a. toxic shock syndrome
b. food poisoning
c. scalded skin syndrome
d. pneumonia
d. pneumonia
The NCCLS recommended procedure for detecting MRSA is
a. the Kirby Bauer disc diffusion method
b. the Vitek automated instrument
c. an oxacillin agar screen test
d. a microdiluion MIC method
c. an oxacillin agar screen test
The usual drug used to treat patients with MRSA infection is
a. vancomycin
b. cephalosporin
c. chloramphenicol
d. gentamicin
a. vancomycin
Penicillinase
a. is a type of beta-lactamase
b. converts penicillin to penicilloic acid
c. inactivates ampicillin
d. a and c are correct
e. a, b, and c are correct
e. a, b, and c are correct
is a type of beta-lactamase, converts penicillin to penicilloic acid, and inactivates ampicillin
May have more than 1 correct answer:
MRSA may be incorrectly identified as CNS because
a. the slide coagulase is negative
b. small, white colonies form on blood agar
c. the tube coagulase is negative at 4 hours
d. the catalase test is negative
a, b, and c
the slide coagulase is negative, small white colonies form on blood agar, and the tube coagulase test is negative at 4 hours
May have more than 1 correct answer:
Tests used for epidemiological studies of MRSA outbreaks include
a. base composition of nucleic acid
b. bacteriophage typing
c. DNA hybridization
d. pulse-field gel electrophoresis
b and d
bacteriophage typing and pulse-field gel electrophoresis
May have more than 1 correct answer:
A tube coagulase should be performed when
a. a staphylococcus agglutinates in saline
b. the slide coagulase is positive
c. the slide coagulase is negative
d. the catalase test is negative
a and c
a staphylococcus agglutinates in saline and the slide coagulase is negative
May have more than 1 correct answer:
Mannitol salt agar
a. is a suitable medium to do a slide coagulase from
b. is a selective medium
c. has a sodium chloride concentration of 0.5%
d. is a differential medium
b and d
is a selective medium and is a differential medium
May have more than 1 correct answer:
The DNase enzyme
a. causes clot lysis
b. is specific for S. aureus
c. breaks down hyaluronic acid
d. depolymerizes DNA
d
depolymerizes DNA
May have more than 1 correct answer:
False negative tube coagulase results may be obtained by
a. incubating for 4 hours at 35 degrees followed by overnight incubation at room temperature
b. using a 1:5 dilution of rabbit serum
c. using plasma contaminated with citrate utilizing bacteria
d. incubating for 48 hours at 35 degrees without checking for clot formation
b and d
using a 1:5 dilution of rabbit serum and incubating for 48 hours at 35 degrees without checking for clot formation
May have more than 1 correct answer:
A typical strain of CNS
a. reduces nitrate to nitrite
b. grows on mannitol salt agar
c. is catalase positive
d. ferments mannitol
a, b, and c
reduces nitrate to nitrite, grows on mannitol salt agar, and is catalase positive
May have more than 1 correct answer:
Most strains of S. aureus are
a. sensitive to oxacillin
b. sensitive to methicillin
c. sensitive to cloxacillin
d. resistant to penicillin
a, b, c, and d
sensitive to oxacillin, methicillin, and cloxacillin and resistant to penicillin
May have more than 1 correct answer:
The following penicillins are classified as beta-lactamase resistant
a. penicillin G
b. oxacillin
c. ampicillin
d. cloxacillin
b and d
oxacillin and cloxacillin
May have more than 1 correct answer:
MRSA
a. grows slower than sensitive strains
b. have an optimum temperature below 37 degrees
c. have both sensitive and resistant populations of cells
d. generally lack free coagulase
a, b, and c
grows slower than sensitive strains, have optimum temperature below 37 degrees, and have both sensitive and resistant strains
May have more than 1 correct answer:
The following adaptations of susceptibility testing are designed to detect resistance of S. aureus to methicillin
a. inspecting zones of inhibition for feathering in toward the disc
b. preparing standardized inoculum directly from colonies
c. adding 2% sodium chloride to MIC broth
d. incubating tests at 37 degrees
a, b, and c
inspecting zones of inhibition for feathering in toward the disc, preparing standardized inoculum directly from colonies, and adding 2% sodium chloride to MIC broth
May have more than 1 correct answer:
Novobiocin susceptibility should be performed on
a. all coagulase positive staphylococci
b. all CNS isolated from blood and spinal fluid
c. all MRSA
d. all CNS isolated from urine in significant amounts
d.
all CNS isolated from urine in significant amounts
May have more than 1 correct answer:
Staphylococcus saprophyticus
a. is catalase and nitrate positive
b. is a common urinary pathogen in young females
c. does not produce free or bound coagulase
d. is susceptible to novobiocin
a, b, and c
is catalase and nitrate positive, is a common urinary pathogen in young females, and does not produce free or bound coagulase
Identify the organism based on the direct smear, colony morphology, and tests results:
Direct smear: GPC in pairs and singles
BA: creamy, opaque, beta
CHOC: creamy, opaque
MSA: growth, mannitol positive (yellow)
Catalase: positive
Slide coagulase: positive
Tube coagulase: positive
Staphylococcus aureus
Identify the organism based on the direct smear, colony morphology, and tests results:
Direct smear: GPC in pairs and singles
BA: white, opaque, beta
MSA: growth, mannitol positive (yellow)
Catalase: positive
Slide coagulase: negative
Tube coagulase: negative
Coagulase negative staphylococcus
Identify the organism based on the direct smear, colony morphology, and tests results:
BA: white, opaque, gamma
Catalase: positive
Slide coagulase: negative
Tube coagulase: negative
Novobiocin: resistant
Staphylococcus saprophyticus
