Microbiology: Pathogens and Bacteriology Overview

Introduction

• Microorganisms can be multicellular, unicellular, or acellular.
• Pathogens are microorganisms that can cause disease.
• Infections are the invasion and multiplication of pathogenic microbes within an individual or population.
• An infection doesn't always result in disease.
• Virulence refers to the degree of pathogenicity.
• Toxin production can be classified as Endotoxin or Exotoxin.
• Endotoxins are part of the outer membrane of Gram-negative bacteria.
• Exotoxins are toxins secreted by bacteria, both Gram-positive and Gram-negative.

Bacteriology

• Gram-positive bacteria include Staphylococci and Streptococci.
• Gram-negative bacteria include:
  • Non-spore forming: Diphtheria, Listeria
  • Spore forming:
    • Aerobic: Bacillus
    • Anaerobic: Clostridium

Staphylococci

• Gram-positive cocci arranged in grape-like irregular clusters.
• Facultative anaerobe.
• All staphylococci are catalase positive.
• Classified into coagulase positive and coagulase negative.
• Coagulase positive: Staphylococcus aureus (S.aureus)
• Coagulase negative (CoNS): Staphylococcus epidermidis and Staphylococcus saprophyticus.

Staphylococcus aureus

• Normal flora in skin and nose.
• 20-50% of humans are carriers.
• Present in the environment (clothing, bed linens).
• Virulence factors:
  • Invasive factors:
    • Adherence factors: Protein A and Clumping factor (Bound coagulase).
    • Enzymes: Coagulase enzyme, Staphylokinase, and DNase.
  • Toxins:
    • Super antigen exotoxins: Exfoliative toxin (SSSS), TSST (Toxic Shock Syndrome Toxin), Enterotoxin
    • Other exotoxins: Leukocidin, Cytolytic exotoxin (hemolysin)

Staphylococcus aureus Virulence Factors

• Protein A: Binds to the Fc portion of IgG, preventing complement activation, inhibiting opsonization, and protecting the organism from phagocytosis.
• Clumping factor (Bound coagulase): Converts fibrinogen into fibrin, depositing it on the cell surface, rendering it resistant to phagocytosis.
• Coagulase enzyme (free coagulase): Converts fibrinogen to fibrin, depositing it on the Staphylococcus surface, protecting the microorganism from phagocytosis.
• Staphylokinase: Fibrinolysin.
• DNase: Depolymerizes DNA in exudates or necrotic tissue.
• Exfoliative toxin: Causes skin desquamation, resulting in Staphylococcal Scalded Skin Syndrome (SSSS).
• TSST (Toxic Shock Syndrome Toxin): Causes fever, hypotension, and rash.
• Enterotoxin (6 heat-stable toxins): Causes food poisoning (vomiting and diarrhea).
• Leukocidin: Kills WBCs.
• Cytolytic exotoxin (hemolysin): Toxic to various cells and RBCs, causing necrosis and hemolysis.

Staphylococcus aureus Pathogenesis

• Invasive Infections:
  • Localized superficial skin infections with pus (pyogenic): Folliculitis, boils, carbuncles, and abscesses.
  • Localized deep infections: Arthritis, osteomyelitis, acute endocarditis.
  • Bacteremia and septicemia.
• Diseases due to toxin production:
  • SSS (Scalded Skin Syndrome): Skin desquamation and generalized bullous formation.
  • TSS (Toxic Shock Syndrome): Predominant in menstruating females using tampons; characterized by fever, skin desquamation, hypotension, and multisystem organ failure.
  • Food poisoning: Self-limiting disease (rapid recovery) caused by preformed enterotoxin ingested in contaminated food; commonly associated with carbohydrates, milk, milk products, cakes, and pastries; incubation period 1-6 hours; symptoms include abdominal colic, nausea, vomiting, but no fever.
• Nosocomial infections: Hospital-acquired infections due to use of catheters and sutures inside hospitals.
• Bacteremia and septicemia.

Staphylococcus aureus Laboratory Diagnosis

• Specimen: Depends on the site of infection, e.g., pus from abscesses or wounds; blood for blood culture.
• Microscopical Examination: Gram-positive cocci, Non-motile, Non-capsulated, Non-spore forming.
• Culture media:
  • Nutrient agar: Produces golden yellow colonies.
  • Blood agar: Produces beta hemolytic colonies
  • Mannitol Salt Agar (MSA)

Staphylococcus aureus Biochemical Reactions

• Catalase test
• Coagulase test
• DNase test
• Phosphatase test

Staphylococcus aureus Typing Methods

• Phage Typing

Staphylococcus aureus Treatment

• Wound drainage.
• Antibiotics:
  • Penicillin.
  • If resistance to penicillin, use Methicillin or Oxacillin.
  • A combination therapy with a penicillinase-resistant penicillin or cephalosporin and clindamycin or a quinolone.
  • Clindamycin, trimethoprim-sulfamethoxazole (TMP-SMX), rifampin, doxycycline, or a quinolone.
  • In case of MRSA Infection: vancomycin or teicoplanin.
  • If resistance to vancomycin (VRSA), use Streptogramins and Linezolid.
  • For topical infection: Mupirocin can be used.

Staphylococcus aureus Case Studies

• Staphylococcus aureus Enterocolitis:

  • In the 1950s and 1960s, S. aureus was a major pathogen in postoperative and antibiotic-associated enterocolitis.
  • A 49-year-old male hospital plumber with a history of hemorrhoids presented with abdominal cramps and diarrhea.
  • Diagnosed with a gastrointestinal infection and was started on levofloxacin, metronidazole, and proton pump inhibitor therapy.
  • Symptoms worsened, and he returned with bloody diarrhea.
  • Computed tomography (CT) scan revealed thickening of the duodenum and jejunum.
  • Enteroscopy revealed an ulcerated duodenum and jejunum with necrosis.
  • Stool microscopic examination and culture were positive for many polymorphonuclear neutrophils and S. aureus.
  • Stool culture was negative for Campylobacter, Shigella, Salmonella, and Yersinia.
  • Exploratory laparotomy revealed a toxic megacolon.
  • Histopathology from the colon and ileum resections showed chronic active colitis with cryptitis and crypt abscesses, focal mucosal inflammation, transmural acute inflammation, ulceration, and necrosis.
  • The patient's symptoms resolved postoperatively and he remained in clinical and endoscopic remission at the 6-and 12-month follow-up visits.

• Staphylococcus aureus native mitral valve endocarditis associated with bed bug bites

  • S. aureus is a leading cause of community-acquired bacteremia and infective endocarditis.
  • S. aureus is a part of the normal skin flora in approximately one third of the human population.
  • Infective endocarditis due to S. aureus can cause several complications and is associated with increased mortality.
  • A 48-year-old female presented with S. aureus bacteremia and native mitral valve endocarditis.
  • Multiple cutaneous skin lesions were identified, reported to be due to recent bed bug bites.
  • No source of infection was found except for the skin lesions.
  • Her hospital course was complicated by pulmonary and cerebral septic emboli, left pleural empyema, and acute renal injury.
  • Bed bug skin bites were suspected as the most likely source of bacteremia.

Staphylococcus aureus Test Yourself

• Compare Exotoxin and Endotoxin.
• Compare Staphylococcus aureus and Staphylococcus epidermidis.
• List the toxins produced by Staphylococcus aureus and their role in pathogenicity.
• Describe the laboratory diagnosis of Staphylococcus aureus.
• Discuss the treatment of Staphylococcus aureus.

This quiz delves into the world of microorganisms, focusing on pathogens, their classifications, and the role of bacteria in disease. Explore topics such as Gram-positive and Gram-negative bacteria, the nature of infections, and the characteristics of staphylococci. Test your knowledge on virulence, toxin production, and more!