Laboratory Diagnosis of Gram-Positive Bacteria: Staphylococci & Streptococci
Sample collection, gram staining, culture techniques, and biochemical tests play crucial roles in the laboratory diagnosis of Gram-positive bacteria like Staphylococci and Streptococci. Understanding these steps is essential for accurate identification and differentiation of these bacteria, aiding in appropriate treatment strategies in medical microbiology.
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MEDICAL MICROBIOLOGY M.S.c Amna shaker Lecture Three Almustaqbal university Department of medical Biotechnology
2 LABORATORY DIAGNOSIS OF GRAM-POSITIVE BACTERIA STAPHYLOCOCCI AND STREPTOCOCCI The laboratory diagnosis of Gram-positive bacteria AND Staphylococci and Streptococci Staphylococci diagnosis involves several steps to identify and differentiate these bacteria. Staphylococci Sample Collection: Samples are collected from the site of infection (e.g., skin, nasal swabs). *Gram Staining: Staphylococci appear as Gram-positive cocci in clusters *Culture Techniques: Blood agar and Mannitol Salt Agar (MSA) are commonly used. S. aureus produces yellow or golden colonies on MSA due to mannitol fermentation
3 Biochemical Tests: 1-Catalase Test: Positive (distinguishes staphylococci from streptococci). 2-Coagulase Test: Positive for S. aureus (differentiates it from coagulase- negative staphylococci). 3-Novobiocin Sensitivity Test: Differentiates S. epidermidis (sensitive) from S. saprophyticus (resistant).
4 gram stain Gram staining is a technique used to differentiate bacterial species into two groups based on the physical and chemical properties of their cell walls The two groups are: 1- Gram-positive bacteria: These bacteria retain the crystal violet stain and appear purple under a microscope. They have a thick peptidoglycan layer in their cell walls. 2-Gram-negative bacteria: These bacteria do not retain the crystal violet stain but take up the counterstain (usually safranin) and appear red or pink. They have a thinner peptidoglycan layer and an outer membrane.
5 Streptococci diagnosis : Sample Collection: Samples are collected from the site of infection (e.g., throat swabs, blood) *Gram Staining: Streptococci appear as Gram-positive cocci in chains or pairs. Culture Techniques: Blood agar is used to observe hemolysis patterns. Hemolysis on Blood Agar: Alpha (partial), Beta (complete), or Gamma (none).
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