Bacteriology exam prep: Rhodococcus, Trueperella, Bacillus

Genus Rhodococcus

• Widespread, environmental bacterium found in soil, particularly in fecally contaminated farm environments
• Rhodococcus hoagii (equi) is the only recognized animal pathogen, causing diseases in horses and humans

Characteristics of Rhodococcus hoagii

• Gram +’ve
• Obligate aerobic
• Non-motile
• Non-spore forming
• Morphologically irregular coccoid to rod shaped
• Facultative intracellular
• Can proliferate inside macrophages
• Coprophilic
• Multiplies in basic pH (8.5-10) herbivore manure and large intestines of foals

Transmission of Rhodococcus hoagii

• Inhalation of airborne particles, particularly in summers when contaminated soil comes desiccated
• Ingestion
• Infected foal's breath may also be a source of infection in advanced cases of pneumonia

Pathogenesis of Rhodococcus hoagii

• Uptake of bacteria into local macrophages at portal of entry
• Alveolar macrophages in the lungs
• Bacteria interferes with phagosome maturation creating a pH neutral vacuole
• Bacterial survival and replication inside pH-neutral vacuole
• Accumulation of replicate bacteria in the macrophage leads to cytotoxicity
• Cell necrosis
• Cascade of proinflammatory signals released, destroying tissues and forming abscesses

Foal Rhodococcosis

• Multifocal purulent bronchopneumonia with abscessation in foals that do not have effective cell-mediated immune response
• Very rarely seen in adult horses
• Exposure to the bacteria soon after birth leads to a gradual development of infection
• Subtle until severe symptoms develop between 4-12 weeks of age
• Other less common manifestations found outside the lungs, spread from primary lesions that are in the lungs or intestines

Other Manifestations

• Osteomyelitis
• Purulent arthritis
• Ulcerative lymphangitis
• Reactive arthritis and ocular inflammatory processes
• Ulcerative typhlocolitis and Mesenteric Lymphadenitis
• Seen in 50% of foals with rhodococcal pneumonia

Rhodococcosis in Other Animals

• Swine and Cattle: Pyogranulomatous lesions in the submandibular, bronchial, and mediastinal lymph nodes, often confused with tuberculosis at slaughter
• Form severe systemic caseating abscesses in the lungs, liver, and other organs in goats and camelids
• Humans: Affects primarily immunocompromised populations, presenting as subacute pneumonia
• Rarely seen in dogs and cats, but associated with immunocompromised patients if reported

Genus Trueperella

• Commensal in skin, respiratory, urogenital, and gastrointestinal tracts of production animals, including cattle, swine, domestic, and some companion animals.
• Opportunistic pathogen that can cause pyogenic infections, including mastitis, arthritis, pneumonia, and abscesses.

Characteristics

• Gram-positive
• Non-motile
• Non-spore forming
• Β haemolytic
• Aerobic and facultative anaerobe
• Coccobacilli with varying shapes and sizes (short rods to coccobacilli)
• Can arrange in singles, pairs, or clusters

Transmission and Infections

• Endogenous infections
• Exogenous transmission via cow-to-cow contact, mediated by flies (e.g., mastitis)
• Often causes infections in combination with other bacteria (e.g., Fusobacterium, Bacteroides, Dichelobactor)
• Local, regional, or metastatic purulent infections in various tissues and organs (e.g., lungs, pericardium, peritoneum, brain, pleura, endocardium, renal cortex, bones, joints, liver, uterus, subcutaneous tissues)

Infections in Cattle

• Metritis and endometritis: T. pyogenes is normal vaginal flora in dairy cattle, opportunistically invading the uterus after parturition and causing infections
• Liver abscesses in feedlot cattle: T. pyogenes normally found in ruminal wall, opportunistically invading through damaged epithelium with F. necrophorum
• Foot rot in feedlot and dairy cattle: necrotic infections of the laminae in the hoof, involving T. pyogenes, F. necrophorum, and P. levii
• Mastitis: severe mastitis with purulent secretions following teat or udder injury, lesions, and dysfunction of the mammary gland

Infections in Other Species

• Pigs: abscesses, pneumonia, pleuritis, endocarditis, osteoarthritis, polyarthritis, mastitis, metritis, septicemia
• Sheep and goats: abscesses in different organs and tissues, including foot abscesses
• Humans: sporadic, zoonotic infections
• Horses, dogs, and cats: rare infections caused by T. pyogenes

Genus Bacillus

• Most species are saprophytes found in air, soil, and water.
• Spores can survive in many habitats and are resistant to heat, radiation, disinfection, and desiccation.
• Can remain dormant in soil for decades.

Subgroups of Bacillus

• Divided into three main groups: B. subtilis, B. cereus, and B. circulans.
• B. subtilis group includes B. subtilis, B. licheniformis, B. pumilis, and B. amyloliquefaciens.
• B. cereus group includes B. anthracis, B. cereus, B. mycoides, and B. thuringiensis.
• B. circulans group includes B. circulans, B. firmus, B. coagulans, and B. lentus.

Characteristics of Bacillus

• Gram-positive.
• Large rods.
• Mostly mesophilic.
• Aerobic or facultatively anaerobic.
• Motile.
• Some exceptions.
• Endospore-producing.

Bacillus Anthracis

• Causes anthrax, which has been reported on nearly every continent, especially in agricultural regions.
• Spores survive in areas with neutral or alkaline calcareous soils.
• Epizootic events are associated with droughts, floods, or natural events that cause soil disturbance and stir up spores.
• Vegetative form is only found in the host body, not in the environment.
• Only spores are found in the environment.

Main Virulence Factors of B. Anthracis

• Toxins:
  • Two toxins encoded in the pXO1 plasmid.
  • Lethal factor (LEF) induces excessive release of cytokines, apoptosis, hypoxia, and cell necrosis.
  • Edema factor (EF) is a highly efficient adenylyl cyclase that converts ATP to cyclic adenosine monophosphate (cAMP).
  • cAMP results in upset water homeostasis in the host cells, leading to massive edema.
  • High concentrations of cAMP result in water efflux.
  • Inactivates phagocytes.
• Capsule:
  • Encoded by the pXO2 plasmid.
  • Plays a central role in virulence by providing strong antiphagocytic protection.
  • Shields the surface structures from the host immune mechanisms.

Secondary Virulence Factors

• B. anthracis is specialized in growing in mammalian hosts and metabolizing peptides, lipids by using peptidases and phospholipases.
• Iron-acquiring peptides and haemolysins serve as secondary virulence factors.

B. Anthracis Transmission and Pathogenesis

• Spores enter the host by skin breach, ingestion, or inhalation.
• Can cause three forms of anthrax:
  • Cutaneous anthrax:
    • Low-level germination at the site of infection.
    • Growth leads to localized edema and necrotic lesions of the skin.
    • Lesions have a coal-black appearance.
    • Rarely described in animals, but if it is, it's usually in pigs.
    • Rarely results in septicemia.

Intestinal Anthrax

• Largely underreported in humans and animals, with immune responses ranging from subclinical infections to death
• Usually acquired through ingestion of infected meat
• Characterized by low-level germination at the site of infection, leading to massive effusion, mucosal edema, and necrotic lesion formation

Intestinal Anthrax Symptoms

• Can present as an oropharyngeal variant, causing fever, toxemia, and inflammatory lesions in the oral cavity
• Enlargement of cervical lymph nodes and edema of the cervical soft tissue are also common symptoms

Pulmonary Anthrax

• The most severe and frequent form in ruminants, associated with herds farmed on dry prairies with short grass where spores are easily inhaled
• Inhaled spores lead to rapid spread into regional lymph nodes, development of hemorrhagic lymphadenitis, septicemia, toxemia, and shock, ultimately resulting in death

Anthrax in Ruminants

• Sudden and unexpected death, possibly preceded by minor anorexia and reduction in milk yield
• Cattle may experience a rapid raise of body temperature (~41.5ºC), staggering, dyspnea, trembling, collapse with convulsive movements, and bleeding from the orifices just before or at the time of death

Anthrax in Other Species

Horses

• Acute, with death typically occurring in 2-3 days following symptom onset
• Symptoms include fever, chills, severe colic, anorexia, depression, weakness, diarrhea, swelling of neck, swelling of abdomen and sternum, and swelling of external genitalia

Pigs

• More resistant to anthrax, but may develop into septicemia
• Acute form characterized by oropharyngitis, progressive swelling of throat, which may result in death by suffocation
• Chronic form characterized by non-specific signs that get better with treatment, including anorexia, vomiting, diarrhea or constipation

Dogs, Cats & Wild Carnivores

• Similar presentation to pigs, but quite resistant to anthrax

Control & Prevention of Anthrax

• Safe disposal and destruction of infected carcasses, with burying or carcasses or infect materials forbidden in several countries
• Culling and burning preferred, with intensive surveillance of animals in contact with infected animals and environment
• Bacteriological diagnosis via blood sample, avoiding opening carcasses to prevent environmental persistence
• Vaccinations are available for prophylactic and proactive action

Other Bacillus Caused Diseases

Bacillus cereus

• Causes rare mastitis in cattle, food poisoning in dogs and cats, and food-borne diarrheal illness in humans

Bacillus licheniformis

• Causes abortion and mastitis in cattle and sheep, and food-borne diarrheal illness in humans

Bacillus subtilis

• Causes ovine abortion, bovine mastitis, and food-borne diarrheal illness in humans