Spatial heterogeneity stabilizes predator-prey interactions at the microscale while patch connectivity controls their outcome.

Natural landscapes are both fragmented and heterogeneous, affecting the distribution of organisms, and their interactions. While predation in homogeneous environments increases the probability of population extinction, fragmentation/heterogeneity promotes coexistence and enhances community stability as shown by experimentation with animals and microorganisms, and supported by theory. Patch connectivity can modulate such effects but how microbial predatory interactions are affected by water-driven connectivity is unknown. In soil, patch habitability by microorganisms, and their connectivity depend upon the water saturation degree (SD). Here, using the obligate bacterial predator Bdellovibrio bacteriovorus, and a Burkholderia prey, we show that soil spatial heterogeneity profoundly affects predatory dynamics, enhancing long-term co-existence of predator and prey in a SD-threshold dependent-manner. However, as patches and connectors cannot be distinguished in these soil matrices, metapopulations cannot be invoked to explain the dynamics of increased persistence. Using a set of experiments combined with statistical and physical models we demonstrate and quantify how under full connectivity, predation is independent of water content but depends on soil microstructure characteristics. In contrast, the SD below which predation is largely impaired corresponds to a threshold below which the water network collapses and water connectivity breaks down, preventing the bacteria to move within the soil matrix.

Bovine subclinical mastitis caused by Mannheimia granulomatis.

Mannheimia granulomatis was isolated for 10 months from the milk of a cow with elevated somatic cell counts. The infection was self-limiting. Phenotypic and molecular characteristics of the isolate were determined.

Assessment of accuracy of disk diffusion tests for the determination of antimicrobial susceptibility of common bovine mastitis pathogens: a novel approach.

A novel approach was used to assess disk diffusion accuracy for determination of antibiotic susceptibility of various bovine mastitis pathogens (Escherichia coli, Staphylococcus aureus, Staphylococcus chromogenes, and Streptococcus dysgalactiae). MIC and disk diffusion diameters were compared for 587 bovine mastitis bacterial isolates collected in Israel and 3,186 drug-organism combinations. Results were analyzed by ROC curves, Bayesian statistics, and standard descriptive methods. Low correlation was observed between results of disk diffusion and MIC for S. dysgalactiae and all antimicrobial agents, S. aureus and erythromycin and neomycin, and E. coli and gentamicin, neomycin, and polymyxin B. On a few occasions in which correlation was satisfactory, accepted susceptibility breakpoints to some of the antimicrobial agents resulted in high discrepancies with MIC results and new breakpoints were suggested-e.g., 21 mm for S. aureus susceptibility to penicillin G instead of 29 mm recommended by the National Committee for Clinical Laboratory Standards (NCCLS) and <21 mm, resistant, 21-25 mm, intermediate, and >25 mm, susceptible for susceptibility of E. coli to trimethoprim/sulfamethoxazole. Thus, this approach enabled determination of the most accurate breakpoints that best fitted the specific prevalence of susceptibility in Israel. Thus, we suggest its adoption by microbiology diagnostic laboratories for the provision of accurate antimicrobial susceptibility results when using the disk diffusion test.