Staphylococcus, Micrococcus, & Similar Organisms - Pt. 2

Staphylococcus Epidermidis

  • Type: Gram-positive cocci

  • Test Results: Positive catalase test, negative coagulase test

  • Colony Characteristics:

    • Size: Very small to medium

    • Color: Gray-white colonies

    • Hemolytic Nature: Nonhemolytic (crucial for identification)

  • Differentiation from Streptococcus:

    • Staphylococcus appears whiter while Streptococcus has a gray appearance

  • Clinical Relevance:

    • Most commonly encountered coagulase-negative staphylococcus

    • Less virulent than Staphylococcus aureus

    • Ubiquitous colonizer and often acts as a contaminant in clinical specimens

  • Infections:

    • Predominantly causes nosocomial (healthcare-associated) infections

    • Introduced into external devices (catheters, heart valves, CSF shunts)

    • Produces a slime layer aiding attachment to prosthetics and evades phagocytosis

    • Significant cause of hospital-acquired urinary tract infections (UTIs)

Staphylococcus Saprophyticus

  • Type: Coagulase-negative Staphylococcus

  • Colony Characteristics:

    • Size: Larger than Staphylococcus epidermidis

    • Appearance: Opaque, white, nonhemolytic

  • Clinical Relevance:

    • Specifically associated with urinary tract infections in sexually active young women

    • Important to consider patient demographics (e.g., less likely in 70-year-old men)

  • Infection Statistics:

    • Second most common cause of urinary tract infections in women after E. coli

    • Even low amounts in urine cultures considered clinically significant

  • Differentiation from Staphylococcus Epidermidis:

    • Novobiocin sensitivity test: resistant if Saprophyticus, susceptible if Epidermidis

Staphylococcus Lugdunensis

  • Classification: Gram-positive cocci

  • Test Results: Positive catalase test, variable coagulase test results

  • Colony Characteristics:

    • Morphology: Appears clumpy, resembling Staphylococcus aureus but often misidentified due to similar morphology

    • Color: Cream to yellow-orange, develops beta hemolysis after 24-48 hours

  • Clinical Relevance:

    • Can cause both community-associated and hospital-acquired infections

    • More virulent than some Staphylococcus species and can mimic Staphylococcus aureus infections

    • Frequently carries mecA gene conferring oxacillin resistance

Differentiation Between Staphylococcus Species

  • Identification Flowchart Overview:

    • Analyze infection source to establish clinical context before identification

    • Gram stain: Identifies gram-positive cocci

    • Catalase test: Positive for staphylococci

    • Slide coagulase test: Interpret results, consider colony morphology and source

  • PYR Test:

    • Recommended for distinguishing Staphylococcus spp. (positive indicates Lugdunensis)

    • Confirmation: Ornithine decarboxylase test to confirm as Staphylococcus lugdunensis

  • Tube Coagulase Test:

    • Positive results indicate Staphylococcus aureus; negative leads to further testing

  • Novobiocin Sensitivity Test:

    • Sensitive results indicate Staphylococcus epidermidis; resistant indicates Staphylococcus saprophyticus

Micrococcus Species

  • Characteristics:

    • Normal flora in humans, rarely pathogenic

    • Gram stain shows tetrads (groups of four cocci)

    • Catalase positive, coagulase negative

    • Bright yellow pigmentation on blood agar media

  • Differentiation from Staphylococcus:

    • Use modified oxidase test (microdase test), positive indicates Micrococcus

Antimicrobial Susceptibility and Resistance

  • Methicillin-resistant Staphylococcus aureus (MRSA):

    • Detrimental infections; resistance mechanisms include altered penicillin-binding proteins (PBP2A)

    • Testing: MRSA screen agar or chrome agar for sensitivity analysis

  • Vancomycin-resistant Staphylococcus aureus (VRSA):

    • Increasing prevalence, especially in patients with underlying conditions

  • D-Zone Test:

    • Determines inducible clindamycin resistance by observing the flattening of the zone between erythromycin and clindamycin discs

    • A flattened area suggests the need for alternative therapies.

Conclusion

  • Understanding how to differentiate between various Staphylococcus species is crucial for diagnosis and treatment in clinical settings.