Antimicrobial Susceptibility of Pasteurella multocida Isolates

The sensitivity to antimicrobial agents of 27 Pasteurella multocida isolates was assayed in terms of minimum inhibitory concentration, employing a serial broth dilution technique. Prepared microtitre plates used in the study contained 17 drugs in four or eight dilutions, with four controls. Isolates of P. multocida were considered in four groups, depending on their origin (from field outbreaks; from carrier animals; from species other than cattle and buffalo; and national reference strains). Despite diverse geographical and host origins, the isolates exhibited uniformity in sensitivity to a majority of the antibacterial agents. For example, all P. multocida isolates were highly sensitive to penicillin, ampicillin, cephalothin, enrofloxacin, chloramphenicol and nitrofurantoin; and all isolates except three were highly sensitive to streptomycin. High resistance to streptomycin was observed in one isolate from a field outbreak; in the 'streptomycin-resistant' marker strain 335 used in Sri Lanka; and in the Thai reference strain. A considerable number of isolates were resistant to fusidin, sulphamethaxozole, spiramycin and c1indarnycin. On the basis of these results, it is suggested that the field practice of administering sulphadimidine to clinically affected animals be discouraged. PASTEURELLA muftocida causes haemorrhagic septicaemia (HS) a highly fatal, septicaemic disease of cattle and buffalo. Since the disease has a sudden onset and a short course, leaving little opportunity for treatment and recovery (De Alwis 1984), attempts to control HS in endemic countries are based on vaccination programs. For chemotherapy to be effective, drugs need to be administered during the early phase of the disease, before specific clinical signs appear (Prescott and Baggot 1988). In HS, this phase is characterised by an elevated temperature and inappetence, increased salivation, and respiratory distress. Treatment may also be of some value in eliminating 'active carriers' that shed pasteurellae intermittently. This study was aimed at determining the susceptibility of P. muItocida isolates, expressed in terms of minimum inhibitory concentration (MIC), to antibacterial agents. Since antibacterial susceptibilities of bacteria are not constant, but vary from time to I Faculty of Veterinary Medicine and Animal Science, University of Peradeniya, Peradeniya, Sri Lanka , Veterinary Research Institute, PO Box 28, Peradeniya, Sri Lanka 193 time and in different environments, it was decided that the P. multocida group from field outbreaks should cover isolates over a 5-year period from different geographical areas of Sri Lanka. Materials and Methods Twenty-seven isolates of P. multocida were tested against 17 antimicrobial agents. All the isolates assayed were held in the freeze-dried collection at Veterinary Research Institute, Peradeniya, Sri Lanka. The four groups of P. multocida comprised: • isolates from field outbreaks of HS from different districts of Sri Lanka over 5 years (1985-1990); • isolates from 'active' and 'latent' carriers of HS collected at abattoirs; • field isolates from species other than cattle and buffalo; • national HS reference strains. The microtitre assay plates (VETMIC + /-, National Veterinary Institute, Uppsala, Sweden) contained 17 antimicrobial agents in eight or four dilution steps, along with four control wells containing trisodium citrate (2), phosphate-buffered saline (1), and distilled water (1). Each time an assay was carried out, a reference strain of Staphylococcus aureus (A TCC 25923) was tested to ensure the stability of the antibacterial ager,ts. P. multocida, being a rapidly growing bacterium, was propagated by inoculating 5 mL brain-heart infusion broth (BHI) with three to five colonies. The inoculum was incubated for 6 hours at 37°C. A 10JLL volume of the bacterial suspension was transferred to 10 mL of BHI to obtain an inoculum density of 1()3-1()4 colony-forming units (CFU) per 50 JLL. Each well of the assay plate was inoculated with 50 JLL of broth culture. After inoculation, plates were sealed with transparent adhesive tape and incubated at 37°C for 18 hours, or longer when required. The plates were placed on a viewer and the opacity in the wells detected in the mirror of the viewer.