Pathogenic Bacteria I & II — Comprehensive Study Notes

Defining Pathogenic Bacteria

  • Pathogen (general definition)

    • Any infectious microorganism able to cause disease once it colonizes a host.
    • Must carry an adequate set of virulence genes (adherence, invasion, immune‐evasion, toxin production, etc.).

  • Categories by requirement to cause disease

    • Obligate pathogen
    • Must infect a host, produce disease, then transmit to a new host to complete its life cycle.
    • Facultative pathogen
    • Capable of infecting/replicating in hosts or multiplying in the external environment.
    • Opportunistic pathogen
    • Normally harmless; becomes pathogenic when host immunity or local conditions are impaired.

  • Categories by cellular location

    • Obligate intracellular
    • Reproduce only inside host cells.
    • Examples: Rickettsia, Ehrlichia, Mycoplasma, Chlamydia.
    • Facultative intracellular
    • Survive both inside and outside cells.
    • Examples: Mycobacterium, Salmonella.
    • Extracellular pathogens
    • Multiply outside host cells, often in tissue fluids or on epithelia.
    • Example: Streptococcus pyogenes (group A).

  • Gram Staining overview

    • Gram-positive: thick peptidoglycan, no outer membrane → purple stain.
    • Gram-negative: thin peptidoglycan + outer membrane → pink stain.
    • Approx. cell size pictured ≈10 μm\approx10\,\mu m (schematic slide reference).

  • Basic morphologic groups mentioned

    • Cocci, Bacilli, Coccobacilli, Curved rods, Spirochetes.
    • Numerous genera listed (Enterobacteriaceae, Pseudomonads, Legionella, etc.).
    • Classification intersects with oxygen requirement (aerobes vs. anaerobes) and intracellular lifestyle.

Gram-Positive Cocci

Genus Staphylococcus

  • Reservoir / ecology
    • Commensal on skin & mucous membranes of humans and animals.
    • Highest carriage in upper respiratory tract (especially nose), lower urogenital, and GIT.
  • Clinically important species
    • S. aureus (humans, many animals), S. pseudintermedius (dogs, cats), S. hyicus (pigs).
  • Pathogenesis
    • Pyogenic (pus-forming) infections → abscesses.
    • Virulence via extracellular fibrinogen-binding protein (clot visualization slide with SEM).
  • Key diseases
    • Pyoderma & Otitis externa (dogs/cats)
    • Often secondary to parasites, allergies, foreign bodies.
    • Any dermatosis that increases skin humidity predisposes to staphylococcal overgrowth.
    • Bovine staphylococcal mastitis (cattle & small ruminants)
    • Mostly S. aureus; contagious; tends to become chronic/subclinical → production losses.
    • Spectrum: peracute gangrenous mastitis ↔ chronic low-grade mastitis.

Genus Streptococcus

  • Reservoir / ecology
    • Commensal on mucous membranes (URT, urogenital). Capsule helps evade phagocytosis.
  • Species & host-specific conditions
    • S. pyogenes (humans).
    • S. agalactiae (cattle) → contagious mastitis (milk ducts).
    • S. dysgalactiae (cattle) → environmental mastitis (skin, buccal cavity).
    • S. equi subsp. equi (horses) → Strangles (not commensal).
  • Strangles (Equine distemper)
    • Febrile URT disease, regional LN abscessation → airway obstruction ("strangling").
    • Highly contagious, transmitted via direct/indirect contact with purulent exudate.
    • Notifiable in many US states.
  • Streptococcal mastitis patterns
    • S. agalactiae → persistent, chronic with acute flare-ups.
    • S. dysgalactiae → acute mastitis; organism resides in buccal cavity, skin, genitalia.

Gram-Positive Bacilli (Aerobic / Facultative)

Listeria monocytogenes

  • Reservoir: Ubiquitous (herbage, sewage, fresh water, feces). Psychrophilic → can grow in refrigerators.
  • Pathogenesis
    • Facultative intracellular; survives in macrophages; hijacks host actin for intracellular motility & cell-to-cell spread.
  • Clinical syndromes (rarely overlap)
    • Septicemia, Abortion, Encephalitis.
  • Ruminant encephalitis ("Circling disease", "Silage disease")
    • Winter–spring; spoiled silage (higher pH) favors multiplication.
    • Affects all ages/sexes; classical circling, cranial nerve deficits.
  • Food safety relevance
    • Major human foodborne pathogen ("when food bites back").
    • Environmental circulation emphasized (diagram slide).

Erysipelothrix rhusiopathiae

  • Reservoir: Tonsils & intestine; ≤50%\le50\% of healthy pigs harbor organism.
  • Susceptible group: pigs 3 mo – 1 yr (stress linked).
  • Four clinical forms
    • Septicemia, Arthritis, Endocarditis, Dermatopathy ("Diamond skin disease" / swine erysipelas).

Corynebacterium pseudotuberculosis

  • Reservoir: Soil; commensal of mucous membranes.
  • Hosts: Sheep & goats.
  • Disease: Caseous lymphadenitis (abscessation & enlargement of superficial/internal LNs).
  • Pathogenesis: Facultative intracellular → survival in macrophages.

Bacillus anthracis

  • Traits: Obligate mammalian pathogen; aerobic/fac. anaerobic; endospore former; soil reservoir.
  • Virulence factors
    • Poly-D-glutamate capsule.
    • Tripartite toxin:
    • Protective antigen (binds host cell).
    • Edema factor (disrupts water homeostasis → edema).
    • Lethal factor (cell death, hypoxia-linked necrosis & hemorrhage).
  • Anthrax in ruminants (peracute septicemia)
    • Rapid death, respiratory distress, bleeding from orifices.
    • Post-mortem: dark unclotted blood, splenomegaly, incomplete rigor.

Gram-Positive Bacilli (Anaerobic)

Genus Clostridium

  • General features
    • Obligate anaerobic, endospore-forming, potent toxins, soil & GI tract reservoir.
  • Pathogenic groups
    • Neurotoxic: affect NMJ without local lesions →
    • C. tetani → Tetanus (spastic paralysis) via tetanospasmin; tetanolysin damages tissue.
    • C. botulinum → Botulism (flaccid paralysis) via pre-formed toxin in feed/carcasses/canned food.
    • Histotoxic: muscle/liver lesions → toxemia (e.g., C. chauvoei Blackleg, C. septicum Malignant edema, C. novyi Black disease).
    • Enteropathogenic/enterotoxemic: intestinal disease & systemic toxemia (e.g., C. perfringens Types A–E, C. difficile → pseudomembranous colitis).

Gram-Negative Bacilli

Order Enterobacterales

  • Reservoir: water, soil, GI tract; includes "coliforms".
  • Major pathogens: Escherichia coli, Salmonella spp., Yersinia spp.
  • Opportunists: Proteus, Klebsiella pneumoniae, Enterobacter aerogenes.
Escherichia coli
  • Ecology: Commensal in GI tract; disease when extra-intestinal.
  • Diseases
    • Neonatal colisepticemia (calves, lambs, poultry)
    • Bloodstream invasion → depression, weakness, tachycardia, hypothermia, diarrhea.
    • Edema disease (weaned piglets)
    • Toxin damages endothelium → perivascular edema; onset 1−21{-}2 weeks post-weaning.
    • Coliform mastitis (cows)
    • Fecal contamination of udder; watery milk w/ white flecks.
Salmonella enterica subsp. enterica
  • Diversity: >2500 serotypes (Typhimurium, Newport, Enteritidis, Dublin, etc.).
  • Reservoir: Many mammals, birds, reptiles; shed in feces.
  • Notable serotypes
    • S. Enteritidis: invades chicken ovaries → inside eggs.
    • S. Dublin (cattle): septicemia, abortion, osteomyelitis; latent carriers (gall bladder) & intermittent shedders.
  • Clinical presentations: Enteritis, septicemia, abortion; many zoonotic.
Brucella spp.
  • Tropism: Reproductive organs of both sexes; intracellular survival in macrophages.
  • Major species & hosts
    • B. abortus (cattle), B. melitensis (sheep & goats), B. suis (pigs), B. canis (dogs).
  • Transmission: Contact with secretions/excretions, inhalation, ingestion, unpasteurized milk.
  • Public health: Severe zoonosis; pasteurization & dairy hygiene critical (slide: life-cycle diagram with dead-end points after pasteurization/fermentation).
Bordetella bronchiseptica
  • Kennel cough (dogs)
    • Co-infection with Canine Adenovirus 1/2; signs: coughing, gagging, oculo-nasal discharge.
    • Intranasal vaccines induce local IgA.
  • Atrophic rhinitis (pigs)
    • Damages nasal mucosa, facilitates toxigenic Pasteurella multocida colonization → turbinate destruction & snout distortion.

Spirochetes

Leptospira spp.

  • Morphology: Gram-negative spirochetes with endoflagella; fragile in dry environments.
  • Ecology: Free-living saprophytes in moist environments; pathogenic spp. persist in renal tubules/genital tract of carriers.
  • Epidemiology: Zoonotic, widespread, outbreaks after floods/hurricanes; noted as underdiagnosed on St Kitts.
  • Animal disease
    • Pigs, cattle, dogs, horses: reproduction failure (abortion, stillbirths) & acute septicemia.
  • Human disease
    • Influenza-like → possible liver/kidney failure.

Borrelia burgdorferi

  • Agent of Lyme disease; vector: Ixodes ticks.
  • Most animal infections subclinical; dogs commonly manifest fever, arthritis, ± cardiac/renal/neuro signs.

Cell-Wall-less Bacteria (Mycoplasma)

  • Key features
    • Smallest free-living prokaryotes; no cell wall → intrinsically resistant to β-lactams (e.g., penicillin).
  • Diseases
    • Cattle: Mastitis, contagious bovine pleuropneumonia, arthritis. Classic posture: head/neck extended, elbows abducted, expiratory grunt, mucopurulent nasal discharge.
    • Swine: Enzootic pneumonia.
  • Hemoplasmas (formerly Haemobartonella)
    • Attach to erythrocytes; uncultivable; cause immune-mediated hemolytic anemia; severity ↑ in splenectomized animals; transmitted by blood & vectors.

Rickettsiales (Obligate Intracellular)

  • Genera & vectors: Rickettsia spp. & Ehrlichia spp.; transmitted mainly by ticks.
  • Diseases
    • Canine cyclic thrombocytopenia → Ehrlichia canis.
    • Human ehrlichiosis (three primary species).
    • Heartwater in ruminants → Ehrlichia ruminantium (regional distribution).

Integrative Themes & Study Tips

  • Intracellular location influences diagnostic approach (e.g., cell culture vs. routine media) and antibiotic choice (e.g., intracellular penetration, β-lactam ineffectiveness vs. Mycoplasma).
  • Many discussed pathogens are zoonotic (Listeria, Salmonella, Brucella, Leptospira, B. burgdorferi, C. botulinum toxins, B. anthracis).
  • Food safety thread: Listeria, Salmonella (eggs), Brucella (raw milk), C. botulinum (canned foods), E. coli (mastitis → dairy contamination).
  • Vector-borne emphasis: ticks (Rickettsiales, Borrelia), mosquitoes rarely for bacteria, but note environmental water for Leptospira.
  • Host predisposition & stress: weaning pigs (E. coli edema), young/stressed pigs (Erysipelothrix), spoiled silage (Listeria), wound necrosis (Clostridium).
  • Toxin vs. invasion dichotomy: neurotoxins (C. tetani, C. botulinum) act distant from bacteria; enterotoxemia (C. perfringens); endotoxin (Gram-negative LPS) plays a role in many septicemias.

Quick Reference Table (mnemonic-friendly)

  • Staphylococcus – Skin (pyoderma), Suppurative abscesses.
  • Streptococcus – Strangles, mastitis in cows.
  • Listeria – Locomotion via actin; Listeriosis (circling, abortion, septicemia).
  • Erysipelothrix – Diamond skin (Emerald rhomboids!).
  • Corynebacterium – Caseous LNs.
  • Bacillus anthracis – Bleeding (unclotted, dark).
  • Clostridium – Convulsions (tetanus) & Canned-food paralysis (botulism).
  • E. coli – Edema disease; mastitis.
  • Salmonella – Serotypes (>25002500) & Shed in feces.
  • Brucella – Birth problems (abortions).
  • Bordetella – Borking cough (kennel).
  • Leptospira – Leptos (abortions) & Lept (kidney/liver) in humans.
  • Borrelia – Borreliosis (Lyme).
  • Mycoplasma – Mysteriously wall-less.
  • Rickettsia/Ehrlichia – Red (blood cells) Effects.

Ethical & Practical Notes

  • Disease notification requirements (e.g., Strangles, Anthrax) tie into public health policy.
  • Proper husbandry (clean bedding, dry skin, feed hygiene) lowers opportunistic infections.
  • Pasteurization, refrigeration, and proper silage pH are critical control points along the food chain.
  • Vaccination strategies: intranasal for Bordetella, toxoid for tetanus.
  • One Health perspective: many veterinary pathogens have direct human repercussions (Salmonella, Brucella, Leptospira), underscoring the veterinarian’s public health role.

Now: Memorize with purpose! Cross-link pathogen traits (Gram, morphology, lifestyle, toxins, host range) to speed recall during exams.