Motility of Bacterial Pathogens in Minimal Media Containing Human Serum

Aims: To investigate the effect of human serum on growth pattern, cellular morphology, and motility of potentially pathogenic bacteria.

Study Design: This was an analytic experimental study.

Place and Duration of Study: Institute of Exact Sciences and Technology (ICET), Federal University of Amazonas (UFAM), Mycology Laboratory, between August 2012 and July 2013.

Methodology: Growth of Escherichia coli, Salmonella typhi, Bacillus subtilis and Bacillus cereus was examined on hard agar (1.5%) solid medium containing 0, 20, or 40 percent pooled human serum. Dilutions of each species were point-inoculated at the center of the plate. Cultivation was carried out aerobically for up to 20 days at 37°C in sealed humidified boxes. Spreading growth was examined by measuring colony diameters, analyzing macro- and micromorphology, and measuring the fractal dimension of colonies.

Results: E. coli and S. typhi strains grew better in Davis and Mingioli agar, whereas B. subtilis and B. cereus grew better in Fujikawa agar. B. cereus and S. typhi developed a white halo of proteolysis around the colony in the medium supplemented with serum. The addition of human serum to minimal hard-agar medium induced a cellular phenotypic change and a colony morphological change, especially in B. cereus and S. typhi. B. cereus and S. typhi developed elongated cells on the colony edge in the presence of human serum, showing cells in raft-like association. Generally, colonies of bacteria grown in the absence of human serum presented smaller fractal and growth dimensions and more-branched spreading, presumably in a sliding translocation.

Conclusion: Cells of “temperate swarmer” species translocated more efficiently on hard agar supplemented with human serum, by sliding and possibly by swarming. The presence of human blood or serum, despite the inhibitory activity of antibodies, may allow pathogenic bacterial cells to overcome the difficulties of low levels of nutrients and hard surfaces with little available water, and may facilitate translocation to other sites. Further investigations of the influence of human serum on swarming and sliding are warranted.