B1.3 Flashcards
Other Staphylococci classifications
S. epidermidis
S. saprophyticus
S. lugdunensis
Other Coagulase-Negative Staphylococci
Methicillin-Resistant Staphylococci aureus (MRSA)
Vancomycin-Resistant Staphylococci
Macrolide Resistance
indigenous microbiota of the skin
contaminant of medical instruments, catheters, CSF shunts and prosthetic heart valve implants (implanted medical devices), hip
prostheses
Staphylococcus epidermidis
Staphylococcus epidermidis disease caused
: Stitch abcess,
Health care-acquired UTIs,
Endocarditis,
Bacteremia
In charge for the adherence of S. epidermidis
Poly-γ-DL-Glutamic Acid (PGA)
LABORATORY DIAGNOSIS used for detecting S. epidermidis
Blood agar plate
Biochemical Test
Susceptibility with 5-ug NOVOBIOCIN (16mm-27mm)
Blood agar plate result for S. epidermidis
gray to white, opaque, small to medium-sized pinheads and non-hemolytic colonies
Biochemical test results for S. epidermidis
Coagulase (-),
CNA (+),
DNase(-),
Mannitol Fermentation (-)
present on the normal skin and in the periurethral and urethral flora
adheres effectively to the epithelial cells lining the urogenital tract
Staphylococcus saprophyticus
Staphylococcus saprophyticus disease caused
common cause of UTI in young sexually active women
significant result for urine culture in order to determine infection by S. saprophyticus
urine culture <10,000 CFU/ml
LABORATORY DIAGNOSIS used to determine S. saprophyticus
Blood agar plate
Biochemical test
Resistance to NOVONIOCIN (5ug; 6mm-12mm) and NALIDIXIC ACID
Absence of Phosphatase production
Blood agar plate result for S. saprophyticus
white, opaque, slightly larger than pin-heads, nonhemolytic colonies although some strains produce yellow pigments
Biochemical Test results for S. saprophyticus
Coagulase (-),
Dnase (-)
Mannitol Fermentation (-)
Staphylococcus lugdunensis results in coagulase test
clumping factor (+),
tube coagulase (-)
contain mecA gene that encodes oxacillin resistance
more aggressive than other CoNS in inefectivity
Staphylococcus lugdunensis
Staphylococcus lugdunensis disease caused
infective endocarditis,
septicemia,
meningitis,
skin and soft tissue infections,
UTIs, and
septic shock
Other Coagulase-Negative Staphylococci that causes endocarditis, septicemia, and wound infections
S. warneri,
S. capitis,
S. simulans,
S. hominis, and
S. schleiferi
Other Coagulase-Negative Staphylococci that causes wounds, bacteremia, endocarditis, and UTIs
medium-sized colonies, with moderate or weak hemolysis and variable pigment production
S. haemolyticus
NOVOBIOCIN SUSCEPTIBLE CoNS
S. epidermidis
S. capitis
S. haemolyticus
S. hominis subsp. hominis
S. lugdunensis
S. saccharolyticus
S. warneri
NOVOBIOCIN RESISTANT CoNS
S. saprophyticus
S. cohnii
S. kloosii
S. xylosus
RESISTANT GENES PRODUCED BY STAPHYLOCOCCI
a. Erythromycin Ribosomal Methylase (ERM) Gene
b. Methionine Sulfoxide Reductase (MSR) A Gene
Class of enzyme inactivating genes
Codes for the methylation of the 23s rRNA
Results in resistance to erythromycin
Inducible or constitutive resistance to clindamycin
May not be detected in routine susceptibility testing
Confer cross-resistance to macrolides (erythromycin) and streptogramins (quinupristin)
Erythromycin Ribosomal Methylase (ERM) Gene
Codes for efflux mechanism
Methionine Sulfoxide Reductase (MSR) A Gene
resistance to erythromycin but susceptibility to clindamycin
efflux mechanism
type of S. aureus that is resistant to methicillin, nafcillin, and oxacillin
acquired after prolonged stay in the hospital, close contact with carriers, effects of broad spectrum antibiotic treatments and exposure to nasal secretions
Methicillin-Resistant Staphylococci aureus (MRSA)
treatment of choice for MRSA
VANCOMYCIN
Methicillin-Resistant Staphylococciaureus (MRSA) types
Community-Associated MRSA (CA-MRSA),
Health Care–Associated Community-Onset (HACOMRSA),
Hospital-Associated (HA-MRSA)
codes for altered Penicillin-Binding
Protein (PBP) = PBP2a or PBP2′
mecA
LABORATORY DIAGNOSIS for identifying MRSA
a. OXACILLIN-SALT AGAR PLATE
b. CHROMOGENIC TEST
c. Latex Agglutination
d. Molecular Nucleic Acid Probes or PCR
Amplification
used to screen for MRSA in clinical samples
differentiate MRSA from hyperproducers of β-lactamase, or Borderline Oxacillin-Resistant Staphylococcus aureus (BORSA)
OXACILLIN-SALT AGAR PLATE
Chromogenic Selective Differential Media:
MRSA Select,
Spectra MRSA,
CHROMagar MRSA
Substance used in chromogenic test that inhibits non-MRSA
CEFOXITIN
Results from CHROMOGENIC test for MRSA
Changes in color of MRSA colonies within 24 to 48 hours using CHROM Agar against colonies of non-MRSA
CHROM agar MRSA colonies color
Mauve-colored colonies
detect altered PBPs
alternative method for testing and confirmation of oxacillin resistance
performed on both CoNS and S. aureus
Latex Agglutination
“gold standard” for MRSA detection
Molecular Nucleic Acid Probes or PCR
Amplification
Vancomycin-Resistant Staphylococci screening used
Vancomycin Agar Plate
discrepant macrolide test results (erythromycin resistant and clindamycin susceptible)
MODIFIED DOUBLE DISK DIFFUSION TEST (D-zone test)
BETA-LACTAMASE TESTS types
Cephalosporinase Test
Acidimetric Method
Iodometric Method
uses cephalosporin or cefinase disk
Substrate: nitrocefin
Cephalosporinase Test
Cephalosporinase Test result for Staphylococci
(+) result: deep pink or red color within 10 minutes (60 minutes for Staphylococci)
Acidimetric Method
Reagent:
pH indicator:
(+) result:
Reagent: citrate-buffered penicillin
pH indicator: phenol red
(+) result: red yellow (penicilloic acid = decrease pH)
Iodometric Method
Reagent:
(+) result:
(-) result:
Reagent: citrate-buffered penicillin and starch iodine complex
(+) result: colorless solution- penicilloic acid reduces iodine and prevents it to combine with starch
(-) result: purple (no color change)8
ANTIMICROBIAL TESTING Treatment
methicillin,
oxacillin,
nafcillin,
cloxacillin, and
dicloxacillin (penicillinase-resistant penicllin drugs)
most commonly used treatment for Antimicrobial testing
Oxacillin
Cutaneous infections treatment used
oral oxaillin or dicloxacillin, if allergic, erythromycin may be substituted
Systemic infections treatment used
parenteral nafcilllin or oxacillin, if allergic,
vancomycin or cephalosporin may be used
MRSA treatment used
vancomcin alone or in a combination with rifampicin
ANTIMICROBIAL TESTING types
A. Screening Test for MRSA: Oxacillin Screen Plate
B. Cefoxitin Disk Diffusion (30 ug)
C. Macro E Test
D. Vancomycin Agar Screen Plate
A. Screening Test for MRSA: Oxacillin Screen Plate culture media used
MHA with 4% NaCl and 6 ug/mL oxacillin
Screening Test for MRSA: Oxacillin Screen Plate procedure
spot inoculated with cotton swab and incubated for 24 hours at 35°C
Microdilution Testing procedure for MRSA screening
oxacilin in cation-supplemented MH broth containing 2% NaCl
Result for Oxacillin Screen Plate for MRSA
Resistant- growth of more than one colony
Susceptible- no growth on the agar plate
Result for Oxacillin Screen Plate for CoNS
Resistant- 24 mm zone of inhibition
Susceptible- >25 mm zone of inhibition
Disadvantage of Oxacillin Screen Plate
Does not reliably detect oxacillin-resistant
CoNS
preferred method for detection of oxacillin-resistance for both S. aureus and S. lugdunensis
Improves detection of MRSA
serves to induce greater PBP2a in mecA-containing strains
Test reagent to detect resistance- both MIC and diffusion method
Cefoxitin Disk Diffusion (30 ug)
Cefoxitin Disk Diffusion (30 ug) interpretation
Resistant- < 21 mm
Susceptible- > 22 mm
detection of heteroresistant VISA because the test uses a higher concentration of organisms (1 x 108 bacteria/ mL)
Macro E Test
best method for detection of either Vancomycin-resistant S. aureus (VRSA) or VISA
Vancomycin Agar Screen Plate
Vancomycin Agar Screen Plate
S. aureus should be screened with
6-ug/mL vancomycin incorporated into BHIA
Confirmatory test for oxacillin resistance
Broth dilution and E-test
