Clinical Microbiology ( MLCM- 201) Prof. Dr. Ebtisam.F. El Ghazzawi. Medical Research Institute (MRI) Alexandria University

outcomes By the end of this lecture the student will able to understand gram positive cocci types, abundance, their pathogenecity and Diagnostic laboratory tests done.

The staphylococci are gram-positive catalase positive spherical cells, usually arranged in grape-like irregular clusters. Staphylococcus aureus Gram stain

They grow readily on many types of media and are active metabolically, fermenting carbohydrates and producing pigments that vary from white to deep yellow. Some are members of the normal flora of the skin and mucous membranes of humans; others cause suppuration, abscess formation, a variety of pyogenic infections and even fatal septicemia. The pathogenic staphylococci often hemolyze blood, coagulate plasma and produce a variety of extracellular enzymes and toxins.

The most common type of food poisoning is caused by heat-stable staphylococcal enterotoxin. Staphylococci rapidly develop resistance to many antimicrobial agents and present difficult therapeutic problems. The genus staphylococcus has at least 30 species. The three main species of clinical importance are Staphylococcus aureus, Staph. epidermidis and Staph. saprophyticus. Staph. aureus is coagulase positive, which differentiates it from the other species.

Staph. aureus is a major pathogen for humans. The coagulase negative staphylococci are normal human flora and sometimes cause infection, often associated with implanted appliances and devices, especially in very young, old and immunocompromised patients. Approximately 75% of these infection caused by coagulase negative staphylococci are due to Staph. epidermidis. Staph. saprophigticus is a relatively common cause of urinary tract infections in young women.

Staph. aureus causes abscesses various pyogenic infections (e.g. endocarditis septic arthritis and osteomyelitis) and food poisoning. It is one of the most common cause of hospital- acquired pneumonia, septicemia and surgical wound infections. It is an important cause of skin infections, such as folliculitis, cellulitis and impetigo. Staph epidermides can cause endocarditis, and prosthetic joint infections. Staph saprophyticus causes urinary tract infections.

a. Specimens: surface swab, pus, blood, tracheal aspirate or spinal fluid for culture. b. Smears: Typical staphylococci are seen in stained smears of pus or sputum. c. Culture: specimens plated on blood agar plates give rise to typical colonies in 24 hrs at 37°C. d. Coagulase test: citrated rabbit or human plasma is mixed with an equal volume of broth culture → coagulation.

e. Catalase test: a drop of hydrogen peroxide solution is placed on a slide and a small amount of the bacterial growth is placed in the solution. The formation of blood bubbles (the releaser of oxygen) indicates positive test. f. Susceptibility testing: Broth microdilution or disk diffusion susceptibility testing should be done routinely on staphylococcal isolates from clinically significant infections.

Assignments Each student will be selected for the assignments s taphylococcus Aureus. Mycobacterium Tuberculosis

Questions DEFINE: The staphylococci. Complete: The pathogenic staphylococci cause …………..,…………. and produce a variety of extra cellular enzymes and toxins.

Recommended Textbooks Manual of Clinical Microbiology, Vols. 1 and 2: Eighth Edition Patrick R. Murray