Chapter 19. Pathogenic Gram-Positive Bacteria. Staphylococcus & Streptococcus

Transcription

1 Chapter 19 Pathogenic Gram-Positive Bacteria Staphylococcus & Streptococcus

2 Staphylococcus Normal members of every human's microbiota Can be opportunistic pathogens Facultative anaerobes Cells occur in grapelike clusters Non-motile Salt-tolerant Tolerate salt on human skin Tolerant of desiccation, radiation, and heat Survive on environmental surfaces

3 Two species commonly associated with diseases in humans Staphylococcus aureus More virulent strain Variety of conditions depending on site of infection Staphylococcus epidermidis Normal microbiota of human skin Opportunistic infections

4 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 1) Structures that enable it to evade phagocytosis 2) Production of enzymes 3) Production of toxins

5 Structural defenses against phagocytosis 1) Protein A coats the cell surface Binds to IgG Inhibits opsonization and the complement cascade 2) Bound coagulase Converts fibrinogen into fibrin molecules Fibrin clots hide the bacteria from phagocytic cells

6 Enzymes 1) Cell-free coagulase: Triggers blood clotting 2) Hyaluronidase: Breaks down hyaluronic acid enabling the bacteria to spread between cells 3) Staphylokinase: Dissolves fibrin threads in blood clots allowing S. aureus to free itself from clots 4) Lipases: Digest lipids allowing staphylococcus to grow on skin and in oil glands 5) -lactamase: Breaks down penicillin allowing bacteria to survive treatment with -lactam antimicrobial drugs

7 Toxins Produced by S. aureus and less frequently by S. epidermidis All are exotoxins 1) Cytolytic toxins: Disrupt the cytoplasmic membrane of a variety of cells 2) Leukocidin: can lyse leukocytes specifically 3) Exfoliative toxins: Cause skin cells to separate and slough off 4) Toxic-shock syndrome toxin: Causes toxic-shock syndrome 5) Enterotoxins: Stimulate symptoms associated with food poisoning

8 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

9 Staphylococcal Diseases Three kinds: 1) Noninvasive disease 2) Cutaneous diseases 3) Systemic diseases Noninvasive disease: Food poisoning due to ingestion of enterotoxincontaminated food Cutaneous diseases: Various skin conditions, Scalded skin syndrome, impetigo, folliculitis

10 Cutaneous diseases Staphylococcal scalded skin syndrome Impetigo

11 Staphylococcal systemic diseases 1) Toxic-shock syndrome (nonstreptococcal) Some Staphylococcus strains produce TSS toxin TSS toxin absorbed into the blood Produces fever, vomiting, rash, low blood pressure

12 The incidence of toxic-shock syndrome in the United States since 1979.

13 Staphylococcal systemic diseases 1) Bacteremia: Presence of bacteria in the blood 2) Endocarditis: Damage to the lining of the heart 3) Pneumonia: Inflammation of the lungs 4) Empsyema: occurs when pus fills the lungs 5) Osteomyelitis: Inflammation of the bone and bone marrow

14 Staphylococcus: Diagnosis, Treatment, and Prevention Diagnosis: Detect Gram-positive bacteria in grapelike arrangements Frequently isolated from pus, tissue exudates, sputum, urine, and blood Cultivation, catalase, biochemical testing, coagulase Treatment: Methicillin, Vancomycin used to treat MRSA infections Prevention: Hand antisepsis important to prevent nosocomial infections

15 Streptococcus Facultative anaerobes 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 Group A Streptococcus: Streptococcus pyogenes Group B Streptococcus: Streptococcus agalactiae α-hemolytic Streptococcus: Streptococcus pneumoniae (Pneumococcus) Streptococcus mutans- causes dental caries

16 Group A Streptococcus: Streptococcus pyogenes Pathogenicity: Structural components to evade phagocytosis 1) M protein destabilizes complement 2) Hyaluronic acid capsule hides bacteria from leukocytes 3) Streptokinases :Break down blood clots, Help Streptococcus spread through tissues 4) Deoxyribonucleases: Reduce the firmness of pus, Facilitate the spread of Streptococcus 5) Hyaluronidase: Facilitates the spread of Streptococcus through tissues 6) Pyrogenic toxins: Can stimulate fever, rash, and shock 7) Streptolysins: Lyse red blood cells

17 Streptococcus pyogenes epidemiology & diseases Epidemiology Typically infects the pharynx or skin Often causes disease when normal microbiota are depleted Spreads via respiratory droplets Diseases 1) Scarlet fever Can occur following streptococcal pharyngitis infections Chest rash develops that spreads across the body

18 2) Pharyngitis ("strep throat") Inflammation of the pharynx Streptococcus pyogenes diseases 3) Pyoderma- pus-producing lesion often on exposed skin 4) Erysipelas infection and inflammation of lymph nodes surrounding a streptococcal infection

19 Streptococcus pyogenes diseases 5) Streptococcal toxic-shock syndrome Bacteremia that causes severe multisystem infections Can cause organ failure, shock, and death 6) Necrotizing fasciitis Streptococci enter the body and spread along the fascia Secrete enzymes and toxins that destroy tissue

20 Streptococcus pyogenes diseases 7) Rheumatic fever Complication of untreated streptococcal pharyngitis Inflammation damages the heart valves and muscle Autoimmune response against heart antigens 8) Glomerulonephritis Caused by antibody-bound streptococcal antigens that accumulate in the glomeruli of the kidneys Inflammation causes hypertension and low urine output Adults may suffer irreversible kidney damage

21 Streptococcus pyogenes diagnosis, treatment & prevention Diagnosis Skin infections diagnosed by the presence of gram-positive bacteria in short chains or pairs Rapid strep test used to diagnose respiratory infections Streptococci in the pharynx are not diagnostic Treatment: Penicillin is effective Prevention: Antibodies against M protein provide protection

22 Group B Streptococcus: Streptococcus agalactiae Pathogenicity Often infects newborns without specific antibodies Produces enzymes whose roles are not yet understood Epidemiology Colonizes the gastrointestinal, genital, and urinary tracts Wound infections and childbirth cause most adult infections Newborns often inoculated at birth

23 Group B Streptococcus: Streptococcus agalactiae Diseases Associated with neonatal bacteremia, meningitis, pneumonia Older immunocompromised patients also at risk Diagnosis, treatment, and prevention ELISA test used to identify group B Streptococcus Treat with penicillin or ampicillin Prophylactic use of penicillin during childbirth reduces newborn infections Immunization of women can protect future children

24 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 One cause of dental caries and dental plaques- Streptococcus mutans Bacteremia can cause meningitis and endocarditis

25 Streptococcus pneumoniae (Pneumococcus) Cocci that most commonly form pairs Form unpigmented, alpha-hemolytic colonies on blood agar Lacks Lancefield antigens Pathogenicity Polysaccharide capsule Protein adhesion: Mediates binding of cells to epithelial cells of pharynx Secretory IgA protease: Destroys IgA Pneumolysin: Lyses epithelial cells

26 Streptococcus pneumoniae (Pneumococcus) Epidemiology Present in the mouths and pharynges of most humans Causes disease when travel to the lungs Infections occur most often in children and the elderly

27 Streptococcus pneumoniae (Pneumococcus) Diseases 1) Pneumococcal pneumonia Most common disease caused by S. pneumoniae 2) Sinusitis and otitis media Sometimes occur following viral infections 3) Bacteremia and endocarditis S. pneumoniae can enter the blood through lacerations or tissue damage 4) Pneumococcal meningitis Mortality rate higher than other causes of meningitis

28 Streptococcus pneumoniae (Pneumococcus) Diagnosis: Gram stain of sputum smears Confirmed with Quellung reaction Treatment: Penicillin, Resistant strains have emerged Prevention: Vaccine made from purified capsular material

29 Enterococcus Previously classified with group D streptococci Reclassified as a separate genus All enterococci live in the intestinal tracts of animals

30 Quellung test Positive quellung or Neufeld reaction: result of the pneumococcal capsular polysaccharide binding with type specific antibody contained in the typing antiserum. An antigen-antibody reaction causes a change in the refractive index of the capsule so that it appears swollen and more visible. After the addition of a counter stain (methylene blue), the pneumococcal cells stain dark blue and are surrounded by a sharply demarcated halo which represents the outer edge of the capsule. The light transmitted through the capsule appears brighter than either the pneumococcal cell or the background.

31 Figure Enterococcus faecalis in lung tissue. Enterococcus

32 Enterococcus Structure and Physiology Form short chains and pairs Lack a capsule Two species cause disease in humans E. faecalis E. faecium Pathogenesis, Epidemiology, and Diseases Found in the human colon Rarely pathogenic at this site Can cause disease if introduced into other parts of the body Important cause of nosocomial infections

33 Enterococcus Diagnosis, Treatment, and Prevention Diagnosis Enterococcus distinguished from S. pneumoniae by its sensitivity to bile Treatment Difficult to treat enterococcal infections Enterococci often resistant to antimicrobials Prevention Prevention is difficult in health care setting Patients often have weakened immune systems Good hygiene and aseptic techniques minimize transmission

34 Make sure you know the organisms inside the red box