Microbial Diseases and Medical Microbiology

Airborne Pathogens

  • most transmitted between people over very short distances

  • Staphylococcus, Streptococcus, and Mycobacterium species (thick cells walls that are more resistant to drying)

  • aerosol droplets person-to-person through coughs or sneezes

  • exploit upper and lower respiratory tracts

    • diseases affecting upper less severe than lower 

  • Streptococcal disease → airborne or by direct contact

    • Streptococcus pyogenes = streptococcal pharyngitis, scarlet and rheumatic fever, necrotizing fasciitis

      • scarlet fever → “strawberry tongue”

      • necrotizing fasciitis → SpeA,B,C,F exotoxins and bacterial surface M protein function as superantigens

      • Rheumatic fever → autoimmune disease; inflammation and tissue damage 

    • Lysogenic phage-encoded streptococcal exotoxin A (SpeA), SpeB, SpeC, SpeF: responsible for streptococcal toxic shock syndrome (TSS)

      • TSS results when T cells secrete cytokines

    • Streptococcus pneumoniae = pneumonia

    • Streptococcal pharyngitis = strep throat

      • symptoms: severe sore throat, enlarged tonsils, red spots of the soft palate, tender lymph nodes, fever, malaise

      • causative agent: S. pyogenes, production of exotoxins

      • diagnosis: rapid antigen detection test, culture of S. pyogenes on blood agar

      • treatment: antibiotics

      • vaccines: no effective vaccines available

      • secondary complications: if left untreated, can lead to scarlet fever, rheumatic fever, acute glomerulonephritis, and toxic shock syndrome

  • Tuberculosis (TB)

    • symptoms: chronic cough, host immune response to TB → formation of macrophage aggregates, primary or secondary infection

    • diagnosis: tuberculin skin test, chest x-ray

    • cause: Mycobacterium tuberculosis, Mycobacterium bovis (less common)

      • Mycobacterium tuberculosis

        • Actinobacteria phylum

        • thick waxy layer of mycolic acid, can’t perform gram stain, need acid-fast staining

        • rough appearance on agar

        • treatment → Isoniazid, Rifampin, daily doses for 2 months

        • vaccines → BCG

  • Leprosy (Hansen’s Disease)

    • transmission → aerosols and direct contact 

    • symptoms: lesions to the body, disfiguring lesions if left untreated, loss of bone calcium (slow shrinking of digits)

    • causative agent → Mycobacterium leprae, can grow inside macrophages

    • treatment → multiple drug therapy for 1 year, Dapsone, Rifampin, Clofazimine

  • Meningitis and meningococcemia

    • symptoms → inflammations of meninges, headaches, stiff neck, vomiting, can lead to coma and death, tissue destruction

    • causative agent → Neisseria meningitidis

    • diagnosis → bacterial culture

    • treatment → penicillin, Rifampin

    • vaccines → Meningococcal B vaccine

  • Cholera 

    • causative agent → Vibrio cholerae

      • need >10^8 cells to cause disease

      • ingested cells attach to epithelial cells in small intestine and release toxins 

      • key virulence factor = cholera toxin CT (forms pentameric CtxB + CtxA complex) → encoded in prophage 

    • symptoms → watery diarrhea, vomiting, massive loss of fluids 

    • transmission → fecal-oral route through contaminated water or shellfish

    • treatment → oral rehydration therapy

    • Cholera epidemics → 7 pandemics occurred, numerous smaller outbreaks

  • Helicobacter pylori and gastric diseases

    • epidemiology → high incidence of transmission within families (direct contact)

    • symptoms → gastritis, stomach ulcers, can lead to stomach cancer

    • causative agent → helicobacter pylori

    • diagnosis → detected by ingestion of radiolabeled urea or biopsy

    • treatment → combination of antimicrobials, antibiotics, and antacids

  • Plague

    • 3 disease forms: 

      • bubonic

      • pneumonic

      • septicemic

    • causative agent → Yersinia Pestis

      • virulence factors: Va nd W antigens prevent phagocytosis, murine toxin causes septic shock, liver damage, and respiratory distress in mice

      • Bubonic form → most common, travels to lymph nodes and cause swelling

      • pneumonic → inhaled directly or reach lungs via lymphatic circulation

      • septicemic → enters blood stream and causes septicemia; local hemorrhages produce dark splotches or cause tissue necrosis

    • transmission → flea bites

    • treatment → antibiotics

    • at least 3 major pandemics

      • 1: early middle ages, 25-50 million deaths

      • 2: 14th-19th century; black death; 75-200 million

        • largest non-viral epidemic

      • 3: 19-20th century; 15 million deaths 

Viral Diseases

Common Cold → infect upper respiratory tract

  • symptoms → rhinitis (inflammation of nasal region)

  • causative agent →

    • Rhinoviruses (~75%) - ss (+) sRNA virus

    • coronaviruses (~15%) - not COVID

    • others 

  • diagnosis → rarely diagnoses

  • treatment → over the counter medication 

Influenza → highly infectious

  • symptoms → fever, headache, malaise, can lead to secondary infections

  • treatment → management of symptoms, antivirals if needed

  • vaccines → yearly vaccinations

  • Antigenic drift: new vaccine prepared each year against major strain    

    • vaccine efficacy reduced next year due to new surface antigens appearing through mutations

    • influenza virus has 8 genome segments

      • re-assortment of viral genome segments of different origins can happen

      • pandemics and outbreaks happen from re-assortments and emergence of new strains

Ebola → direct contact with blood or bodily fluids

  • symptoms → fever, weakness, headaches, diarrhea, vomiting

  • causative agent → ebola virus

  • treatment → currently no drugs, only supportive care to alleviate symptoms

  • vaccines → several in development

Dengue Fever → transmitted by mosquitoes

  • causative agent → dengue virus

  • symptoms:

    • asymptomatic

    • mild - high fever, headache, joint pain, rashes

    • severe - dengue hemorrhagic fever

  • diagnosis: tourniquet test

  • treatment → supportive care

  • no current vaccines

BSL-4 Labs: highly specialized labs to handle most dangerous pathogens

  • workers wear full-body suits with outside air supply/ventilation and air locks control all access to labs

Clinical Specimen Collection and Handling

Step 1: identify the most likely suspects

  • observing disease symptoms in patients

  • knowing which organism produces such symptoms

  • being aware of similar disease outbreak nearby

  • identifying the etiological agent (crucial)

    • use appropriate treatments

    • anticipate likely sequelae (sore through could signal mild infection)

    • track spread of disease

Biosafety Containment Procedures

Algorithms: step-by-step problem solving procedures developed by clinical microbiologists to expose the most likely cause of a given infectious disease

  • binary yes/no questions about the clinical speciment

  • biochemical tests

  • serological tests

Selective Media: inhibits the growth of certain groups of organisms; allows initial selection and growth of pathogen

Differential media: facilitates identification by confirming characteristic biochemical traits