Analyses of the Distribution Patterns of <i>Burkholderia pseudomallei</i> and Associated Phages in Soil Samples in Thailand Suggest That Phage Presence Reduces the Frequency of Bacterial Isolation

<div><p>Background</p><p><i>Burkholderia pseudomallei</i> is a soil saprophytic bacterium that causes melioidosis. The infection occurs through cutaneous inoculation, inhalation or ingestion. Bacteriophages (phages) in the same ecosystem may significantly impact the biology of this bacterium in the environment, and in their culturability in the laboratory.</p><p>Methods/Principal Findings</p><p>The soil samples were analysed for the presence of bacteria using culture methods, and for phages using plaque assays on <i>B</i>. <i>pseudomallei</i> strain 1106a lawns. Of the 86 soil samples collected from northeastern Thailand, <i>B</i>. <i>pseudomallei</i> was cultured from 23 (26.7%) samples; no phage capable of infecting <i>B</i>. <i>pseudomallei</i> was detected in these samples. In contrast, phages capable of infecting <i>B</i>. <i>pseudomallei</i>, but no bacteria, were present in 10 (11.6%) samples. <i>B</i>. <i>pseudomallei</i> and their phages were co-isolated from only 3 (3.5%) of soil samples. Since phage capable of infecting <i>B</i>. <i>pseudomallei</i> could not have appeared in the samples without the prior presence of bacteria, or exposure to bacteria nearby, our data suggest that all phage-positive/bacteria-negative samples have had <i>B</i>. <i>pseudomallei</i> in or in a close proximity to them. Taken together, these findings indicate that the presence of phages may influence the success of <i>B</i>. <i>pseudomallei</i> isolation. Transmission electron microscopy revealed that the isolated phages are podoviruses. The temperate phages residing in soil-isolated strains of <i>B</i>. <i>pseudomallei</i> that were resistant to the dominant soil borne phages could be induced by mitomycin C. These induced-temperate phages were closely related, but not identical, to the more dominant soil-isolated phage type.</p><p>Conclusion/Significance</p><p>The presence of podoviruses capable of infecting <i>B</i>. <i>pseudomallei</i> may affect the success of the pathogen isolation from the soil. The currently used culture-based methods of <i>B</i>. <i>pseudomallei</i> isolation appear to under-estimate the bacterial abundance. The detection of phage capable of infecting <i>B</i>. <i>pseudomallei</i> from environmental samples could be a useful preliminary test to indicate the likely presence of <i>B</i>. <i>pseudomallei</i> in environmental samples.</p>