Pathogenic Gram-Positive Bacteria - Lesson 8 PDF

Pathogenic Gram-Positive Bacteria- part 1 Cecilia Garlanda Gram-Positive Bacterial Cell Walls Thick layer of peptidoglycan containing TEICHOIC ACIDS and lipoteikoic acids (polysaccharides) anchoring the peptidoglycan to the cytoplasmic membrane. Negative charge. Mycobacterium: mycolic acid, a wax...

Pathogenic Gram-Positive Bacteria- part 1 Cecilia Garlanda Gram-Positive Bacterial Cell Walls Thick layer of peptidoglycan containing TEICHOIC ACIDS and lipoteikoic acids (polysaccharides) anchoring the peptidoglycan to the cytoplasmic membrane. Negative charge. Mycobacterium: mycolic acid, a waxy lipid, that protects from desiccation. Difficult staining in water-based dyes, so acid-fast stain is used. The thick cell wall of a Gram-positive bacterium retains crystal violet dye: purple. 2 Gram-Positive Bacterial Pathogens • Stain purple when Gram stained • Two major groups based on DNA – Low G + C bacteria: <50% guanine-cytosine base pairs – High G + C bacteria: >50% guanine-cytosine base pairs Low G + C Gram-Positive Bacteria Three genera of cocci: Staphylococcus Streptococcus Enterococcus Three genera of bacilli: Bacillus Clostridium Listeria Mycoplasmas Staphylococcus • Normal members of every human's microbiota • Can be opportunistic pathogens Staphylococcus Grapelike cluster Staphylococcus • Structure and Physiology – Facultative anaerobes – Cells occur in grapelike clusters – Nonmotile, 0.5-1 micron in diameter – Catalase-positive – Salt-tolerant • Tolerate salt on human skin – Tolerant of desiccation, radiation, and heat • Survive on environmental surfaces Staphylococcus • Structure and Physiology – Two species commonly associated with diseases in humans (out of 45 known species) • Staphylococcus aureus – More virulent species – Variety of conditions depending on the site of infection • Staphylococcus epidermidis – Normal microbiota of human skin – Opportunistic infections Staphylococcus • Pathogenicity – Infections result when staphylococci breach body's physical barriers – Entry of only a few hundred bacteria can result in disease – Pathogenicity results from three features • Structures that enable it to evade phagocytosis and innate immune response • Production of enzymes • Production of toxins External Structures of Bacterial Cells 1- Glycocalyx- Glycocalyces: =sweet cup polysaccharides, polypeptides. Gelatinous, sticky. Capsule: organized repeating units of chemicals firmly attached to the cell’s surface. Slime layer: loose, water-soluble glycocalix. Staphylococcus Functions: Favouring attachment to surfaces as BIOFILM. Protection from desiccation. Cause of disease: Protection from recognition by phagocytes (e.g. Streptococcus pneumoniae, Klebsiella pneumoniae). Bacteroides • Pathogenicity Staphylococcus – Structural defenses against phagocytosis • Protein A coats the cell surface (S. aureus) – Binds to IgG (Fc portion) – Inhibits opsonization and the complement cascade • Bound coagulase (S. aureus) – Converts fibrinogen into fibrin molecules – Fibrin clots hide the bacteria from phagocytic cells • Synthesize polysaccharide capsules (S. aureus and S. epidermidis). Different serotypes with different virulence – Inhibit leukocyte chemotaxis and phagocytosis – Facilitate attachment of Staphylococcus to surfaces (e.g. catheters, shunts, synthetic joints) Staphylococcus • Pathogenicity – Enzymes (S. aureus) • Cell-free coagulase – Triggers blood clotting (only S. aureus, diagnostic!) • Hyaluronidase – Breaks down hyaluronic acid – Enables the bacteria to spread between cells • Staphylokinase – Dissolves fibrin threads in blood clots – Allows S. aureus to free itself from clots Staphylococcus • Pathogenicity – Enzymes • Lipases – Digest lipids – Allow staphylococcus to grow on skin and in oil glands • b-lactamase – Breaks down penicillin – Allows bacteria to survive treatment with b-lactam antimicrobial drugs Mechanisms of Resistance 1- Resistant microbes can produce an enzyme that deactivates the drug. E.g. betalactamase or penicillinase brakes beta-lactam rings. 200 beta-lactamases have been identified, often located on R plasmids. Staphylococcus • Pathogenicity – Toxins: Produced by S. aureus and less frequently by S.epidermidis • Cytolytic toxins – Disrupt the cytoplasmic membrane of a variety of cells (4 toxins) – Leukocidin can lyse leukocytes specifically: release of enzymes • Exfoliative toxins* – Cause skin cells to separate and slough off because disrupt desmosomes. Proteases. • Toxic-shock syndrome toxin* – Causes toxic-shock syndrome • Enterotoxins*: heat stable (100°C for 30 min). 18 proteins. – Stimulate symptoms associated with food poisoning. *Can act as superantigens, causing massive production of cytokines by macrophages (IL-1, TNFa) and T cells (IL-2, IFNg). Staphylococcus • Epidemiology – S. epidermidis is ubiquitous on human skin – S. aureus primarily found only in moist skin folds – Both species grow in the upper respiratory, gastrointestinal, and urogenital tracts of humans – Transmitted by direct contact or fomites – Handwashing and aseptic techniques help prevent infections Staphylococcus • Staphylococcal Diseases Depend on site of infection, immune state of host, toxins and enzymes secreted by specific strains. Ubiquitous. -Toxin activity -Bacteria invasion and proliferation: abscesses and tissue destruction – Noninvasive disease • Food poisoning – Due to ingestion of enterotoxin-contaminated food. One of the more common causes of food poisoning. Processed food contaminated with bacteria from human skin, room temperature, bacteria reproduce and secrete toxin. Heat stable. Resistant to stomach and jejunum. Normal taste. Symptoms (nausea, vomiting, diarrhea..) in 4 hs, last for 24 h. – Cutaneous diseases • Various skin conditions: pyogenic lesions: impetigo, folliculitis, wound infections • Scalded skin syndrome (toxin) Impetigo Small, flattened, red patches on the face and limbs that develop in pus-filled vesicles (bacteria and cells). S. aureus is cause of 80% of impetigo cases. Folliculitis Infection of hair follicle: red, swollen, pus filled. Furuncle: extension into surrounding tissue. When several furuncles coalesce, they form a carbuncle which extends deeper into tissues: fever. Here the infection reached the lymph node Staphylococcal scalded skin syndrome Reddening of the skin, near the mouth, then over entire body. Large blisters containing clear fluid, without bacteria and leukocytes, since lesions are caused by toxins and not by bacteria or inflammation (important for diagnosis). In two days, outer layer of skin peels off in sheets. No scarring. SECONDARY BACTERIAL INFECTIONS!! Also called Ritter disease. Neonates and young children. Staphylococcal scalded skin syndrome Staphylococcus • Staphylococcal Diseases – Systemic diseases: potentially fatal. • Toxic-shock syndrome (non-streptococcal). RARE – Some Staphylococcus strains produce TSS toxin – TSS toxin absorbed into the blood » Produces fever, vomiting, rash, low blood pressure, multiorgan failure, loss of sheets of skin. 5% fatal: shock Toxic-shock syndrome (TSS) Red rash is associated with internal organ damage The incidence of toxic-shock syndrome in the United States since 1979 Epidemic of TSS among menstruated women in 1980. S. aureus grows well in superabsorbent tampons. Staphylococcus • Staphylococcal Diseases – Systemic diseases (S. aureus, epidermidis and others) • Bacteremia – Presence of bacteria in the blood. 50% are nosocomial infections: surgical wounds, medical devices. Often the initial site of infection (focal infection) is not identified. • Endocarditis – Damage to the lining of the heart. 50% lethal. • Pneumonia – Inflammation of the lungs – Empyema occurs when pus fills the lungs • Osteomyelitis – Inflammation of the bone and bone marrow • Urinary tract infections in young women Staphylococcus • Diagnosis, Treatment, and Prevention – Diagnosis • Detect Gram-positive bacteria in grapelike arrangements – Treatment: resistance to penicillin has spread during 20th century. 5% strains are susceptible today. • Methicillin: semisynthetic, not inactivated by b-lactamase. MRSA strains are emerging, they are resistant to several antimicrobial drugs. Emerging disease: communityassociated MRSA (CA-MRSA). • Vancomycin used to treat MRSA (methicillin resistant) infections. VRSA strains are emerging. – Prevention • Hand antisepsis important to prevent nosocomial infections. Cleansing of wounds. Draining of abscesses. Streptococcus • Facultative anaerobes, catalase-negative • Cocci arranged in pairs or chains • Often categorized based on Lancefield classification – Divided into serotypes based on bacteria's antigens – Lancefield groups A and B include the significant human pathogens Streptococcus • Group A Streptococcus (GAS): Streptococcus pyogenes Colonies: 1-2 mm in diameter, large zone of beta-hemolysis in 24h culture in bloodagar plates. - Differential media: one component is used in a differential manner by diverse microorganisms. e.g. - blood agar: Streptococcus pneumoniae partial digests blood: alphahemolysis; Streptococcus pyogenes completely digests: betahemolysis; Enterococcus foecalis does not digest: gamma-hemolysis. Blood agar Streptococcus • Group A Streptococcus: Streptococcus pyogenes Colonies: 1-2 mm in diameter, large zone of beta-hemolysis in 24h culture in bloodagar plates. – Pathogenicity • Structural components to evade phagocytosis – M protein destabilizes complement – Hyaluronic acid capsule hides bacteria from leukocytes • Streptokinases – Break down blood clots – Help Streptococcus spread through tissues • Deoxyribonucleases – Reduce the firmness of pus – Facilitate the spread of Streptococcus Streptococcus • Group A Streptococcus: Streptococcus pyogenes – Pathogenicity • C5a peptidase – Breaks down complement protein C5a – Decreases movement of white blood cells to the infection site • Hyaluronidase – Facilitates the spread of Streptococcus through tissues • Pyrogenic or erythrogenic toxins* (on temperate phages)-> – Acting on macrophages and T cells, cause release of cytokines that stimulate fever, rash, and shock • Streptolysins – Lyse red and white blood cells, and platelets *superantigen The lysogenic replication cycle in bacteriophages 1 Attachment 3 Prophage in chromosome 2 Entry Lambda phage Lytic cycle Lysogeny 6 Synthesis 8 Release 4 Replication of chromosome and virus; cell division 7 Assembly 5 Induction Infected host cells grow and reproduce normally for generations before they lyse Further replications and cell divisions Streptococcus • Group A Streptococcus: Streptococcus pyogenes – Epidemiology • Typically infects the pharynx or skin…eventually deeper tissues • Often causes disease when normal microbiota are depleted or adaptive immunity (Abs) is impaired • Spreads via respiratory droplets • Before discovery of antimicrobials, was cause of death of several people. Streptococcus • Group A Streptococcus: Streptococcus pyogenes – Group A streptococcal diseases • Pharyngitis ("strep throat") – Inflammation of the pharynx: red, swollen, purulent abscesses Pharyngitis Pus pockets on tonsils Streptococcus • Group A Streptococcus: Streptococcus pyogenes – Group A streptococcal diseases • Pharyngitis ("strep throat") – Inflammation of the pharynx: red, swollen, purulent abscesses • Scarlet fever (scarlatina) – Can occur following streptococcal pharyngitis infections – Chest rash develops that spreads across the body – Caused by pyrogenic toxins released by lysogenized strains Scarlet fever (scarlatina) Streptococcus • Group A Streptococcus: Streptococcus pyogenes – Group A streptococcal diseases • Pyoderma and erysipelas – Pyoderma-pus-producing lesion often on exposed skin. Due to direct contact with an infected patient or contaminated fomites. Superficial. Highly contagious. – Erysipelas infection and inflammation of lymph nodes surrounding a streptococcal infection. Deep. Typically on the face of children. Erysipelas Localized pus-filled lesions Streptococcus • Group A Streptococcus: Streptococcus pyogenes – Group A streptococcal diseases • Streptococcal toxic-shock syndrome – Bacteremia that causes severe multisystem infections. Rare, in HIV+ patients, or with other diseases. – Can cause organ failure, shock, and death (40%). • Necrotizing fasciitis – Streptococci enter the body and spread along the fascia – Secrete enzymes and toxins that destroy tissue. Associated to toxemia, organ failure, death (50%). Necrotizing fasciitis “Flesh-eating” group A Streptococcus pyogenes Streptococcus • Group A Streptococcus: Streptococcus pyogenes – Group A streptococcal nonsuppurative diseases • Rheumatic fever – Complication of untreated streptococcal pharyngitis – Inflammation damages the heart valves and muscle – Autoimmune response against heart antigens • Glomerulonephritis – Caused by antibody-bound streptococcal antigens that accumulate in the glomeruli of the kidneys: disease caused by immune complexes – Inflammation causes hypertension and low urine output – Adults may suffer irreversible kidney damage Streptococcus • Group A Streptococcus: Streptococcus pyogenes – Diagnosis, treatment, and prevention • Diagnosis – Skin infections diagnosed by the presence of grampositive bacteria in short chains or pairs (normally not a member of skin microbiota) – Rapid strep test used to diagnose respiratory infections (group A streptococcal antigens) » Streptococci in the pharynx are not diagnostic • Treatment – Penicillin is effective • Prevention – Antibodies against M protein provide only strain specific protection: the infection can occur more than once – antibiotic prophylaxis Streptococcus • Group B Streptococcus: Streptococcus agalactiae – Cocci that form chains – Distinguished from group A Streptococcus • Group-specific polysaccharide cell wall antigens • Smaller zone of beta-hemolysis • Resistant to bacitracin Streptococcus • Group B Streptococcus: Streptococcus agalactiae – Pathogenicity • Often infects newborns without specific antibodies • Produces enzymes (proteases, hemolysins, deoxyribonuclease, hyaluronidase) whose roles are not yet understood – Epidemiology • Colonizes the gastrointestinal, genital, and urinary tracts • Wound infections and childbirth cause most infections • 60% of newborns inoculated at birth: most are protected by maternal Abs. If not: early (<7days) or late (>7 days) onset. Once 50% mortality. Streptococcus • Group B Streptococcus: Streptococcus agalactiae – Diseases • Associated with neonatal bacteremia, meningitis, pneumonia. 3/1000 newborns. 5% mortality; meningitis results in neurological damage, eg deafness, blindness, mental retardation (25% cases). • Once: puerperal fever (postpartum endometritis). • Older immunocompromised patients also at risk (25% mortality) – Diagnosis, treatment, and prevention • ELISA test used to identify group B Streptococcus • Treat with penicillin or ampicillin, vancomicin • Prophylactic use of penicillin during childbirth reduces newborn infections. Mothers (urinary tract) are tested before delivery. • Pregnant women: screening for colonization • Immunization of women could protect future children Streptococcus • Other Beta-Hemolytic Streptococci – Streptococcus equisimilis • Causes pharyngitis – Streptococcus anginosus • Infection produces pus-filled abscesses – Penicillin effective against both species Streptococcus • Alpha-Hemolytic Streptococci: The Viridans Group – Lack group-specific carbohydrates • Cannot be grouped by Lancefield system – Many produce a green pigment when grown on blood media – Inhabit mouth, pharynx, GI tract, genital tract, and urinary tract – Opportunistic pathogens: pus-filled abdominal abscesses – One cause of dental caries and dental plaques (use dextran to colonize enamel) – If enter the blood can cause meningitis and endocarditis Dental caries Streptococcus • Streptococcus pneumoniae – Cocci that most commonly form pairs – Form unpigmented, mucoid, alpha-hemolytic colonies on blood agar – Lacks Lancefield antigens – 92 different strains, based on caspule antigens, collectively called pneumococci – Member of pharyngeal microbiota: opportunistic pathogen Streptococcus pneumoniae Capsule Once called Diplococcus Streptococcus • Streptococcus pneumoniae – Pathogenicity: not completely understood • Polysaccharide capsule: protects from digestion. Required for virulence. • Teichoic acid and peptidoglycan fragments: activation of C, TLRs, NLRs-> inflammation • Phosphorylcholine – Stimulates cells to phagocytize the bacteria: bacteria hide inside body cells • Protein adhesin – Mediates binding of cells to epithelial cells of pharynx • Secretory IgA protease – Destroys protective IgA • Pneumolysin – Lyses epithelial cells upon binding to cholesterol and interferes with lysosome activity Streptococcus • Streptococcus pneumoniae – Epidemiology • Present in the mouths and pharynges of most humans • Causes disease when travels to the lungs • Infections occur most often in children and the elderly Streptococcus • Streptococcus pneumoniae – Pneumococcal diseases • Pneumococcal pneumonia – Most common disease caused by S. pneumoniae – S. pneumoniae is the most common cause of pneumonia • Sinusitis (all ages) and otitis media (children) – Sometimes occur following viral infections. Pus production • Bacteremia and endocarditis – S. pneumoniae can enter the blood through lacerations or tissue damage • Pneumococcal meningitis – Mortality rate higher than other causes of meningitis Streptococcus • Streptococcus pneumoniae – Diagnosis, treatment, and prevention • Diagnosis – Gram stain of sputum smears – Confirmed with Quellung reaction: anticapsular Abs cause the capsule to swell. – Culture is difficult because of fastidious nutritional requirements. Colonies are lysed by bile. • Treatment – Penicillin » Resistant strains have emerged • Prevention – Vaccine made from purified capsular material Streptococcus pneumoniae Protein adhesins