Gram negative bacteria

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Gram negative bacilli

Growth on MacConkey

Pseudomonads

Psudomonas

Burkholderia

Aeromonas

Curved rods

Campylobacter*

Lawsonia**

Helicobacter*

Vibrio

Enterobacteriaceae

Escherichia coli

Salmonella

Shigella (human pathogen)

Yersinia

No growth on MacConkey

Respiratory pathogens

Pasteurella

Haemophilus

Actinobacillus

Bordetella***

Legionella

Other mucosal pathogens

Brucella

Taylorella

Moraxella

Francisella

Anaerobes

Bacteroides

Fusobacterium

Dichelobacter

* Requires special growth conditions

** Uncultivatable in vitro

*** Grows on MacConkey

Gram negative cocci are human pathogens only (Neisseria, Moraxella). Note Moraxella can cause pink eye in cattle.

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Pseudomonads

Pseudomonads include:

Pseudomonas aeruginosa (the ultimate opportunist)

Burkholderia mallei (glanders in horses - exotic to Australia)

Burkholderia pseudomallei (melioiis or pseudoglanders - endemic to northern Australia)

Aeuromonas hydrophila (causes disease in lower vertebrates including birds, reptiles, frogs, and fish)

Pseudomonas aeruginosa

Pseudomonas aeruginosa is known as the green pus bacteria. It is a common pathogen of high veterinary importance.

It is an environmental organism, living in water, soil, skin, and the intestinal tract (transient). It is a opportunistic pathogen, entering through wounds, devitalised tissue, burns, ear canals, or the respiratory tract (cystic fibrosis patients). Infection is associated with immunosuppression or tissue damage.

P. aeruginosa is a formidable pathogen that possesses an array of virulence factors and widespread resistance to antimicrobials. It is ubiquitous and infects many species including humans.

The virulence factors of Pseudomonas aeruginosa include:

Polar flagellum (motility)

Quorum sensing (sensing and communicating with other cells - allows for formation of biofilms/aggregates)

Pili (adherence to glycoproteins on epithelial cells for attachment)

Endotoxin (released once bacterial is dead - shock)

Polysaccharide (slime - antiphagocytic, resists antimicrobial penetration, helps with adherence)

Siderophores (sequests Fe++ from hosts - requires for cell division)

Various exotoxins (released from viable bacteria)

Exotoxin A (inhibits host defences, lethal

Haemolysin (toxic to alveolar macrophages and causes ciliastasis)

Phospholipase C (hydrolysis of lecithin)

Pigments (antibacterial agents)

Proteases (causes local tissue necrosis and spreading)

Exoenzyme S (local and systemic toxicity)

Leukocidin (inhibits host defences by damaging the cytoplasma of neutrophils)

Pseudomonas forms biofilms. Individual bacteria communicate via quorum sensing, allowing them to facilitate the movement of nutrients in and waste out, promote spread, etc. Biofilms are very hard to remove, as they are protected from the immune system and antibiotics.

Pseudomonas is also very resistant to many antimicrobials. This is due to intrinsic resistant (outer membrane impermeability, efflux pumps, and enzymes) as well as acquired resistance (beta-lactamases and 'promiscuous' sharing of R-plasmid encoded and chromosomal resistance genes.

Diseases associated with Pseudomonas infection are many and varied.

Integument: otitis externa, wound infections, fleece rot, mastitis, eye infections, and necrotic stomatitis (especially reptiles)

Respiratory system: pneumonia, lung abscesses

Also metritis, urinary tract infection, septicaemia

Diagnosis: specimens to take include pus, respiratory aspirates, urine, mastitic milk, ear swabs... depending on the location of disease.

Pseudomonas is aerobic, with characteristic growth on blood agar (pigment, corn chip odour, most are beta haemolytic. It grows on MacConkey agar (lactose negative). Colonies are large, flat, and grayish-blue.

Pseudomonas are oxidase positive. P. aeruginosa grows at 42 degrees celcius.

PCR and molecular tests (for strain typing) is also available.

In vitro susceptibility testing should be performed on all isolates to determines drug of choice for treatment, as Pseudomonas is resistant to many antimicrobials. Perform surgical debridement to remove any biofilms.

Aminoglycosides in combination with a beta-lactam can be used in combination. Fluoroquinolones and ticarcillin/clavulanic acid (Timentin) be used in more resistant infections.

Ear infections can be treated wiwth timentin, animoglycosides (ototoxic - check tympanic membrane), enrofloxacin (Baytril Otic), or polymixin B (Surolan). Cleaning the ear with Tris-EDTA before applying the medication significantly lower MIC (punches holes in the outer membrane to allow for easier penetration of drugs).

Burkholderia mallei

Burkholderia mallei causes a disease called Glanders (farcy), a systemic pyogranulomatous disease. It is a mostly a disease of antiquity and effects solipeds (horses, mules, etc.). It is zoonotic.

Transmission is in contaminated food, water, fomites, and sometimes through inhalation and wounds. The incubation period is days to weeks.

Burkholderia mallei is a true parasite - it is unable to survive for extended periods in the absence of a host.

Glanders is characterised by granulomatous lesions of lungs, nose, and skin. An acute septicaemic form that affects donkeys and mules is fatal within weeks. A protracted chronic and subclinical form affecting horses that will about 50% of infected animals (without antimicrobials). This form makes horses the primary carriers.

There are three cardinal signs of glanders that can occur individually or in combination:

Chronic nasal discharge (+/- ulceration of the nasal septum)

Enlargement and induration along the lymphatics and lymph nodes, especially submandibular lymph nodes

Presence of nodules, pustules, or ulcers on the flanks and extremities of infected animals

In endemic areas, clinical signs are adequate for diagnosis.

Laboratory diagnosis must be performed with care as this is a biohazard. Specimens to take include discharges from lesions, tissue, or blood for serology. Culture can only be performed in certain reference labs. Serology, fluorescent antibody assays, and immunohistochemistry can also be performed.

PCR is becoming a more common method of diagnosis.

The Strauss reaction (in which male pigs are injected with infected clinical material) is not performed anymore.

A field test (the Mallein test) has been used to eradicate the disease from Europe and North America (test and slaughter policy. The disease is still endemic in Eastern Europe, Turkey, and parts of Asia. It is a notifiable disease in Australia.

Burkholderia pseudomallei

Burkholderia pseudomallei causes a disease called melioidosis.

This bacteria is a soil and water dweller. It lives in the clay layer of the soil and reported cases increase during the wet season. It is endemic to central America, Asia, and northern Australia.

Melioidosis occurs in a wide variety of hosts, including goats and sheep, pigs, camels, alpacas, cattle, and native fauna.

Transmission is via inhalation, through cuts on the skin, and occasionally ingestion. It has also occurred transplacentally in goat and possibly through arthropod bites.

The incubation period is 1-21 days, but latent infections occur, with disease sometimes manifesting decades after exposure.

B. pseudomallei is a an oppurtunist. Abscesses form in internal organs (lungs, spleen, liver, joints, and CNS) and soft tissues. The clinical signs of disease are dependent upon the location of the abscesses in the body.

In equine, symptoms resemble glanders (pseudoglanders).

Disease is more common in immunocompromised animals.

In Queensland, many otherwise healthy feral pigs are condemned at abattoirs due to B. pseudomallei abscesses.

Disease in humans manifests are acute pulmonary infection, and abscess or granuloma formation. Some cases may also be septicaemic. Disease cases are usually sporadic with occasional clusters due to contaminated water (high rainfall). Risk factors for humans include diabetes mellitus, renal failure, alcoholism, and rheumatic heart disease.

Mellioidosis is difficult to diagnose clinically.

Care needs to be taken with laboratory diagnosis as this bacterium is a biohazard.

Diagnosis is similar to glanders, with samples taken from lesions or blood. Culture is performed in reference labs, and serology, fluorescent antibody tests, and immunohistochemistry are available. Molecular methods (PCR on buffy coat or purulent exudates) are common.

There is no effective method of prevention in endemic regions due to B. pseudomallei being an environmental organism.

Treatment includes surgical drainage of abscesses and parenteral antimicrobial therapy. Drugs that can be used include tetracycline, chloramphenicol, TMS, amoxicillin-clavulanic acid, third generation cephalosporins, and carbapenems.

There are no effective vaccines available.

Aeromonas hydrophila

Aeromonas hydrophila affects aquatic organism with a wide variety of disease manifestations. These include:

Fin/tail rot in fish

Red leg disease in amphibians

necrotic stomatitis in reptiles

Aeromonas hydrophila requires a low temperature for growth (30 degrees celcius).

It also causes occasional disease in mammals (diarrhoea, opportunistic infections). Outbreaks in people occurs from consuming contaminated food or water.

Treatment is with terramycin (a tetracycline).