Characterization of Biofilm Formation in [Pasteurella] pneumotropica and [Actinobacillus] muris Isolates of Mouse Origin.

[Pasteurella] pneumotropica biotypes Jawetz and Heyl and [Actinobacillus] muris are the most prevalent Pasteurellaceae species isolated from laboratory mouse. However, mechanisms contributing to their high prevalence such as the ability to form biofilms have not been studied yet. In the present investigation we analyze if these bacterial species can produce biofilms in vitro and investigate whether proteins, extracellular DNA and polysaccharides are involved in the biofilm formation and structure by inhibition and dispersal assays using proteinase K, DNase I and sodium periodate. Finally, the capacity of the biofilms to confer resistance to antibiotics is examined. We demonstrate that both [P.] pneumotropica biotypes but not [A.] muris are able to form robust biofilms in vitro, a phenotype which is widely spread among the field isolates. The biofilm inhibition and dispersal assays by proteinase and DNase lead to a strong inhibition in biofilm formation when added at the initiation of the biofilm formation and dispersed pre-formed [P.] pneumotropica biofilms, revealing thus that proteins and extracellular DNA are essential in biofilm formation and structure. Sodium periodate inhibited the bacterial growth when added at the beginning of the biofilm formation assay, making difficult the assessment of the role of ß-1,6-linked polysaccharides in the biofilm formation, and had a biofilm stimulating effect when added on pre-established mature biofilms of [P.] pneumotropica biotype Heyl and a majority of [P.] pneumotropica biotype Jawetz strains, suggesting that the presence of ß-1,6-linked polysaccharides on the bacterial surface might attenuate the biofilm production. Conversely, no effect or a decrease in the biofilm quantity was observed by biofilm dispersal using sodium periodate on further biotype Jawetz isolates, suggesting that polysaccharides might be incorporated in the biofilm structure. We additionally show that [P.] pneumotropica cells enclosed in biofilms were less sensitive to treatment with amoxicillin and enrofloxacin than planktonic bacteria. Taken together, these findings provide a first step in understanding of the biofilm mechanisms in [P.] pneumotropica, which might contribute to elucidation of colonization and pathogenesis mechanisms for these obligate inhabitants of the mouse mucosa.

Comparison of the in vitro susceptibility of rodent isolates of Pseudomonas aeruginosa and Pasteurella pneumotropica to enrofloxacin.

The objectives of this study were to determine and compare the in vitro enrofloxacin susceptibility of 94 Pseudomonas aeruginosa isolates obtained from enrofloxacin-treated and untreated mice and that of 40 Pasteurella pneumotropica strains and also to assess the efficacy and effects of enrofloxacin treatment of laboratory mice. The minimum inhibitory concentrations (MICs) of enrofloxacin against all the Ps. aeruginosa isolates were in the range of 1 to 4 microg/ml, whereas those against all the P. pneumotropica strains were less than 0.5 microg/ml. The mutation frequency in 54% of the Ps. aeruginosa isolates on treatment with enrofloxacin ranged from 10(-6) to 10(-8); however, none of the P. pneumotropica strains could grow on medium containing more than 3 microg/ml enrofloxacin. Comparison of in vitro enrofloxacin susceptibilities suggested that enrofloxacin was effective for eliminating P. pneumotropica but not for eliminating Ps. aeruginosa for which the MIC of enrofloxacin was more than 1 microg/ml. These results indicated that the enrofloxacin susceptibility of P. pneumotropica was higher than that of Ps. aeruginosa, and that the enrofloxacin treatment might not affect the susceptibility of Ps. aeruginosa.