Inhibition of swarming motility using in vitro hyperthermia.

Hyperthermia is a therapeutic technique in which body tissue is exposed to temperatures in the region of 40-45 °C to induce a physiological or biological effect. Swarming motility is an important virulence factor for Proteus mirabilis and Pseudomonas aeruginosa and swarming phenomenon is a coordinated multicellular movement of differentiated bacterial population over semi-solid surfaces. In this study, we aimed to investigate the inhibitory effect of hyperthermia on bacterial swarming motility using a modified thermobiogram method and show the potential of this thermal method to treat bacterial infections. Ten P. mirabilis and 10 P. aeruginosa clinical isolates were included in the study. Sheep blood agar (SBA) plates were prepared and inoculated with bacterial suspensions of clinical isolates. Inoculated SBA plates were incubated inside 2 different incubators; at 37 °C and 45 °C for 20 h. The diameter of bacterial growing zones (swarming diameters) were measured every 2 h and noted. Finally, Gram stains of the isolates were prepared for microscopic examination. Wilcoxon signed-rank test was used to compare the swarming inhibition rates of the isolates incubated at 37 °C and 45 °C. Regarding P. mirabilis species, a significant difference was found between two different temperatures (P = 0.0078). So, a temperature at the level of hyperthermia significantly inhibited the swarming motility of P. mirabilis isolates. In addition, transformation to coccus form was observed at 45 °C. We speculate that these findings might be useful for employing thermal therapies including hyperthermia method to treat infectious diseases caused by swarming bacterial pathogens in the future.
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