LECT. 5.pdf

Medical Microbiology: Lecture-5 Pathogenic Bacteria Gram-positive cocci There are two medically important genera of gram-positivecocci: Staphylococcus and Streptococcus. 1. Staphylococcus: I- General features: 1. Staphylococcusare G+ve cocci (spherical or grapes shape)....

Medical Microbiology: Lecture-5 Pathogenic Bacteria Gram-positive cocci There are two medically important genera of gram-positivecocci: Staphylococcus and Streptococcus. 1. Staphylococcus: I- General features: 1. Staphylococcusare G+ve cocci (spherical or grapes shape). 2. Are non-motile, non-capsulated, non-spore forming. 3. Staphylococcusare oxidase negative & catalase positive which one feature that distinguishes from Streptococci. 4. Staphylococci are part of normal flora of human skin, nose, respiratory and gastrointestinal tracts. Are also found in air, dust and other in human environments. 5. Staphylococcus has at least 30 spp., three spp of clinical importance are Staphylococcus aureus (S. pyogenes), S. epidermidis (S. albus), S. saprophyticus (S. citrus). II- Transmission: S. aureus is major pathogenic spp for human. Transmission of bacteria from human to human by inhalation of respiratory secretion or consumption of contaminated food.Sta. epidermidis is found primarily on the human skinand can enter the bloodstreamat the site of intravenouscatheters that penetrate through the skin.Sta. saprophyticusis found primarily on the mucosa of the genital tract inyoung women and from that site can ascend into the urinarybladder to cause urinary tract infections. III- Pathogencity & clinical features: A: Pathogenesis Pathogenic virulence factors are enable an organism to produce disease. The clinical outcome of an infection depends on the virulence of the pathogen and the opposing effectiveness of the host defense mechanisms. Pathogenesis of S. aureus depends on the combined actions of several virulence factors, so it is difficult to determine precisely the role of any given factor. S. aureus expresses many potentialvirulence factors: ❖ Cell wall virulence factors (Protein A and protein F). ❖ Cytolytic exotoxins or hemolysins (α, β, γ, δ toxin). 1  ❖ Super-antigen exotoxins:{Enterotoxins (six major antigenic types: A, B, C, D, E, and G) and Toxic shock syndrome toxin (TSST-1)} B: Clinical significance, Staphylococcal infections are classified as: 1. Skin infections; such as abscesses, pyoderma (impetigo), furuncles, carbuncles, boils, folliculitis, cellulites, toxic shock syndrome, and scalded skin syndrome. 2. Respiratory tract infections; such as tonsillitis, pharyngitis, sinusitis, pneumonia, and Otitis media. 3. Other infections; endocarditis, osteomyelitis, meningitis, and nosocomial infections, surgical-wound infectionsit is the mostcommon cause of bacterial conjunctivitis. 4. Food poisoning (Staphylococcal gasteroenteritis) is due to enterotoxin, which characterized by short incubation period (1-6 hr.) after consumption contaminated food. Vomiting, diarrhea, nausea, never fever are most symptoms. IV- Laboratory diagnosis: A: smear examination, stained smear shows G+ve cocci arranged in cluster. B. culture of S. aureus, the sample is plated on blood agar, showing yellow colonies with Beta hemolytic. Identifications of bacteria is confirmed by catalase positive, coagulase test positive, mannitol fermentation, and grow in high concentration (7.5%) of NaCl. V- Control 1. Treatment:Sta. aureus strains are resistant to penicillin G. Most of these strains produce a-lactamase. oxacillin, vancomycin, cephalosporins, and ciprofloxacin.Treatment witha combination of a β-lactamase–sensitive penicillin (e.g.,amoxicillin) and a β-lactamase inhibitor (e.g., clavulanicacid) is also useful. 2. Prevention: a: cleanliness, hygiene and antiseptic management of lesion can control the spread of S. aureus. B: there are no effective vaccines against S. aureus. 2Streptococcus: I- General features: 1. Streptococci are G+ve cocci (spherical, chain or pairs shape). 2. Are non-motile, non-spore formingand non-capsulated (some strain have capsule). 1. Streptococci are oxidase & catalase negative which one feature that distinguishes the Streptococci from Staphylococci. 2. Streptococci are member of normal flora skin, respiratory tract and some are normal flora ofenteric and genital tracts of human. 2 3. A streptococcus has at least 20 spp. S. pyogenes, and S. pneumoniaeare clinical importance for human. II- Transmission: Respiratory tract infections (S. pyogenes) are transmitted by inhalation of respiratory droplets. Skin infection occurs after direct contact with infected individuals or contaminated fomites. III- Pathogencity & clinical features: A: Pathogenesis S. pyogenes cells, perhaps in an inhaled droplet, attach to the pharyngeal mucosa via actions of protein F and M protein. The bacteria may simply colonize. Alternatively, bacteria may grow and secrete toxins, causing damage to surrounding cells, invading the mucosa, and eliciting an inflammatory response with attendant influx of white cells, fluid leakage, and pus formation. The patient then has streptococcal pharyngitis. Occasionally, there is sufficient spread that the bloodstream is significantly invaded, possibly resulting in septicemia and/or seeding of distant sites, where cellulitis (acute inflammation of subcutaneous tissue), fasciitis (inflammation of the tissue under the skin that covers a surface of underlying tissue), or myonecrosis (death of muscle cells) may develop rapidly or insidiously.S. pyogenesexpresses many potential virulence factors: ❖ Capsule (hyalouronic acid). ❖ Cell wall virulence factors (Protein M, and protein F). ❖ Extracellular products; hemolysins or (streptolysin O, S), streptokinase, strepto doranses, and hyaluronidase. ❖ Super-antigen exotoxins:{ Streptococcal shock syndrome toxin (SSST)} B: Clinical features Streptococcal infections are classified as: 1. Pyogenic infections (skin & respiratory tract infection) ❖ Skin infections; A: Erysipelas: red area has rapid spread to give a butter fly distribution with blister on skin of face. B: Puerperal sepsis: this infection is following the delivery of newborn. C: Cellulitus: erythema, swelling, pain lesion of skin and subcutaneous tissues. The infection is associated with burns, wound or surgical incisions. ❖ Respiratory tract infection; A: Sore throat (tonsillitis) after incubation periods (1-3 days), or it may be invade pharynx and causes pharyngitis. 3  B: It may be causes severe pneumonia with fever and cough. 2. Toxigenic infections: Scarlet fever; it is caused by ethrogenic toxin; the lesion is associated with pharyngitis or skin or soft tissues infection. The lesion is characterized by fever, vomiting, rash, peeling of skin, and strawberry tongue. The rash appears on trunk after 24hr. of illness. 3. Immunogenic disorder; the infections occur after (1-3 weeks) from acute infections. A: Rheumatic fever (RF): is occurs after upper respiratory tract infection (URTI) with sore throat not after skin infection. RF is caused by autoantibodies that react with heart muscle due to similarity in structure. B: Acute Glomerulonephritis (AGN); is one of most common complications of URTI and skin infection. AGN is immune complex disease, which lead damage of kidney. IV- Laboratory diagnosis: A: Microscopic smear: smear shows G+ve cocci arranged in chines or pairs. B. culture of S. pyogenes,the sample is plated on blood agar, showing small colonies with Beta hemolytic. Identifications of S. pyogenes is confirmed by Bacitracin sensitivity test. C: Serological test: are a rise in titer of antibodies against group A streptococci can be estimated such as antistreptolysin-O (ASO), if ASO titer in streptococci serum is excess of 160-200 units is indicate streptococcal infection especially RF. V- Control 1. Treatment: penicillin G, aminoglycoside, erythromycin and ciprofloxacin. Antimicrobial drug have no effect on AGN and RF. 2. Prevention: A: personal hygiene and eradication from carriers. B: there are no effective vaccines againstS. pyogenes but specific M proteins vaccine is being tested. 4