Pasteurella multocida is a Gram-negative, nonmotile, penicillin-sensitive coccobacillus from the family Pasteurellaceae. P. multocida is the cause of a range of diseases in mammals and birds, including fowl cholera in poultry, atrophic rhinitis in pigs, and bovine hemorrhagic septicemia in cattle and buffalo. It can also cause a zoonotic infection in humans, which typically is a result of bites or scratches from domestic pets. Many mammals (including domestic cats and dogs) and birds harbor it as part of their normal respiratory microbiota, rendering them silent sources of infection.
History Pasteurella multocida was first found in 1878 in cholera-infected birds. However, it was not isolated until 1880, by Louis Pasteur, in whose honor Pasteurella is named.
Taxonomy
Subspecies P. multocida is traditionally divided into three subspecies by biochemical phenotype:
P. multocida subsp. multocida is defined as dulcitol(-), sorbitol(+). P. multocida subsp. septica is defined as dulcitol(-), sorbitol(+). P. multocida subsp. gallicida is defined as dulticol(+). A variant classification replaces the ambiguous Andrades sorbitol growth test with a test for α-glucosidase (α-Glu) activity. It agrees well with single-primer (M13 core) PCR fingerprinting but not so well with the original sorbitol test. This biochemical classification is generally used for epidemology. The different ability to digest these sugar alcohols may have a meaning in ecology and pathogenesis.
Serology Strains of the species are serologically categorized by a combination of two designations: one of five Carter serogroups (A, B, D, E, F) based on the capsule and one of 16 Heddleston serovars (1–16) based on lipopolysaccharide (LPS). To classify a strain serologically, the capsule and the LPS are separately tested using pre-made antibodies. The two results are then written in a combined serotype (e.g. A:2, C:1), for a total of 80 possibilities. (It is also possible for the capsule to not be present, written as "-".) The chemical structure for A, D, and F capsules are known to be hyaluronic acid, heparin, and chondroitin respectively; group B is known to contain some copolymer of arabinose, mannose, and galactose. The chemical structure for LPS of all 16 serovars are known, as are many types not included in the Heddleston system. Another indication system is Namioka-Carter, commonly encountered in bovine literature. The capsule classification is the same as Carter's but there are instead 11 somatic "O groups" again supposedly corresponding to the LPS. Results are written with the serovar first, e.g. 6:B and 6:E. Namioka's method for LPS resolution (tube agglutination) is not as powerful as Heddleston's gel diffusion precipitation test but may be easier to perform. There is not a good correspondance between any pair of Namioka and Heddleston numbers, suggesting at least some of the antisera in one of the systems is not as purely descriptive of LPS as originally thought.
Molecular typing The genetic locus responsible for the capsular serological variation is known, so "molecular serotyping" (i.e. deducing the serotype by the genotype) has become routine and in the overwhelming majority of cases gives identical results, generally only producing mismatches on novel combinations of mutations. The locus for the LPS is also known, but only six LPS genotypes are defined due to some LPS serovars being produced from very minor genetic change that are hard to differentiate by PCR fingerprinting. They are designated L1 (1, 14), L2 (2, 5), L3 (3, 4), L4 (6), L5 (9), L6 (10, 11, 12, 15), L7 (8, 13), L8 (16), with the parenthezied numbers being the corresponding serovars. Again, the two designations are usually combined: for example, two strains with serotypes A:1 and A:14 should both test as having the genotype A:L1. Two multilocus sequence typing schemes have been defined, an RIDIC scheme based on fragments of 7 housekeeping genes (adk, est, gdh, mdh, pgi, pmi, g6pd "zwf") originally defined for avian isolates and a multihost scheme based on different fragments of 7 housekeeping genes (adk, aroA, deoD, gdhA, g6pd, mdh, pgi). The former scheme has 365 genotypes (ST1–265, ST for "sequence type") and the latter has 109 (ST1–109); to differentate between those two one usually prefix a designation like "RIRDC ST365", etc. Because the two schemes use overlapping areas, a ST from one scheme usually only corresponds to a handful in another, though no strict conversion table can be made. MLST can be used in addition to the capsule:LPS genotype as these systems are orthogonal. Because MLST uses housekeeping genes unrelated to virluence, it is not subject to as much selection pressure and tends to more closely show the history of the strains as measured by more intensive whole-genome comparisons.
Disease Pasteurella multocida causes a range of diseases in wild and domesticated animals, as well as humans. The bacterium is found in birds, cats, dogs, rabbits, cattle, and pigs. In birds, P. multocida causes avian or fowl cholera disease; a significant disease present in commercial and domestic poultry flocks worldwide, particularly layer flocks and parent breeder flocks. In most species the majority of infections are caused by a handful of serotypes, partly explained by the fact that the two molecules used for serotype classification are also the host's immune system's "first impression" of this bacterium. However, capsule and LPS are only two of the bacterium's virulence factors with a role in host selection, and two strains of the same cap:LPS genotype may actually turn out quite different when examined using MLST or whole-genome comparison.
Avians P. multocida strains that cause fowl cholera in poultry typically belong to the serovars 1, 3, and 4. In the wild, fowl cholera has been shown to follow bird migration routes, especially of snow geese. The P. multocida serotype-1 is most associated with avian cholera in North America, but the bacterium does not linger in wetlands for extended periods of time.
Mammals P. multocida causes atrophic rhinitis in pigs; it also can cause pneumonia or bovine respiratory disease in cattle. It may be responsible for mass mortality in saiga antelopes.
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