Pasteurella are gram-negative coccobacilli or rods that are facultative anaerobes and normal flora of the respiratory tracts of many animals. Pasteurella multocida commonly causes infections in animals and can infect humans through animal bites or contact. P. multocida is the most frequent human isolate and causes wound infections, cellulitis, bone/joint infections, and respiratory infections. Identification involves gram staining, culture on blood agar showing gray colonies within 24 hours, and biochemical tests showing oxidase and urease positive results.

1. PASTEURELLA
Sanjeev Regmi
M.Sc. Medical Microbiology
Tribhuvan University, Nepal

2. Introduction
 Non-motile, gram negative, facultative
anaerobic coccobacilli or rods
 Causes chicken cholera and haemorrhagic
septicaemia in various animals and birds
 Most important species include: P. multocida, P.
pneumotropica, P. aerogen, P. dagmatis, P.
canis, P. stomatis, P. caballi, P. bettyae
 Most species are oxidase positive, catalase
positive and alkaline phosphatase positive and
reduce nitrate to nitirite

3.  Most species produce acid form glucose, fructose,
mannose ands sucrose and do not hydrolyse starch or
salicin, produce urease
 Pasteurella and animal associatedActinobacillus are
difficult to distinguish phenotypically (both are catalase
+ve, oxidase +ve, gram –ve coccobacilli that reduce
nitreate to nitrite and also produce urease)
 P. multocida:- non motile, gram –ve facultative
anaerobic rods or coccobacilli (0.7x0.4µm) with bipolar
staining.
- Non sporing, capsulated in culture at 37 deg celsius
- Oxidase and indole positive , Ornithine decarboxylase
+ve, urease +ve
- Fails to grow in MA but grow on BA

4. Culture characteristics:
 Grow well on CA and sheep BA forming
smooth gray colonies that are 0.5-0.2mm in
diameter at 35-37deg celsius after 24hrs of
incubation
 Non haemolytic
 Do not grow on MA and other types of
selective media and differential enteric media
 A characteristic odour(like E.coli but more
pungent) is noted, perhaps due to formation
of large amount of indole by organism

5. Virulence factors:
 Capsule: some strains of P. multocida are
capsulated
 Dermonecrotic toxin: essential virulence factor
for development of progressive respiratory tract
infection, also activates several cellular signal
transduction pathways. ‘tox A’ gene is
responsible for production of toxin which is
present in conserved region of P. multocida
chromosome
 Production of lipases
 Broad host range

6. Pathogenesis:
 They are usually commensals of respiratory tract of
animals like rabbits, rats, horses, sheep, fowl, dogs,
cats
 Most infection in human are acquired from animal
contact
 Human infections are rare
 Localised cellulitis and lymphadenitis occurs after
cat or dog bite or scratch
 Infection of respiratory system: bronchitis,
pneumonia, empyema
 Systemic infections including meningitis in
immunocompromised patients

7. Clinical significance:
 P.multocida is most frequently recovered Pasteurella species
from human specimens and is also recovered from a wide
variety of animals. In some animals it causes shipping fever and
haemorrhagic septicaemia in cattle
 Cholera in fowl
 Atrophic rhinitis in swine
 Pleuritis, pneumonia, abscess formation, otitis
media,septicaemia in laboratory rabbits
 Human infections include:
 Wound infections characterized by rapid development of pain,
erythema, swelling, cellulitis with or without abscess formation
 Bone and joint infection
 Liver diseases occur in immunocompromised host

8. Lab diagnosis:
 Specimens: swab from wounds, CSF in case
of meningitis, secretions or sputum in case of
respiratory infections
1. Direct microscopy: gram staining reveals
gram –ve coccobacilli or rods (0.7 X 0.4µm)
2. Culture: specimens cultured on BA and CA.
Smooth, large butterfly gray colonies after
overnight incubation at 35-37deg celsius
3. Biochemical tests: indole +ve, oxidase +ve
urease +ve, Ornithine decarboxylase +ve

9.  Antimicrobial susceptibility testing indicates P.
multocida are susceptible to penicillin,
ampicillin, amoxycillin-davaulanate, expanded
spectrum cephalosporins (cefotaxime,
cefuroxime, ceftazidime), tetracycline and
chloramphenicol
 Less activity has been noted with first
generation cephalosporin and semi synthetic
penicillins
 Resistant to vancomycin and clindamycin

10. Other species:
 P. pneumotropica: it is a part of normal flora of
respiratory tract of dogs, cats, rats and mice. In lab
rodents, it causes lower respiratory tract infections
and abscesses. Human infections caused by P.
pneumophila include meningitis, bacteremia with
shock, borne and joint infections, wound infection,
cellulitis, pnumonia etc
 P. aerogens: it is a part of oropharyngeal and
intestinal flora of swine.This organism is also
associated with abortion and still birth in animals
such as pigs, dogs, rabbits. P. aerogens was isolated
from ear and throat of still born child.

11.  P. canis and P. stomatis: isolated from wound
infections resulting from dog bites. Causes wound
infection in human host
 P. dagmatis: normal flora of oral cavity of dogs and
cat and is associated with bites and scratches from
these animals. Human infections caused by P. dagmatis
include cellulitis, a groin abscess, a throat abscess and
rarely causes pneumonia, bacteremia and endocarditis
 P. bettyae: reservoir of P. bettyae is currently
unknown. It has been recovered from amniotic fluid,
placenta, blood of newborn babies, surgical incisions,
leg abscesses. In a cluster of five patients with urethritis
PID or Bartholins gland abscesses was identified in USA
in which P. bettyae was isolated as etiologic agent. It
suggests that P. bettyae may be a sexually transmitted
pathogen.