Physicochemical factors affecting the growth of Burkholderia pseudomallei in soil microcosm

Abstract

Burkholderia pseudomallei causes melioidosis, the third most common cause of death from infectious diseases in northeast Thailand. Four physicochemical factors were set so that their values covered the range of the northeast, which is an endemic area. The soil pH was set at pH 4-10, soil salinity was 0.0-5.0% NaCl, total iron was 50-150 mg/kg soil, and carbon to nitrogen ratio (C/N) was 10:1 to 40:1. The experiments were carried out at 37°C, and soil moisture was maintained for 7 days. The number of viable bacterial cells was counted daily. Soil pH, salinity, Fe, and C/N ratio affected the bacterial growth. The bacterial colony was significantly (P < 0.05) reduced at soil pH > 8, soil salinity > 1% NaCl, and C/N ratio > 40:1. However, the growth of B. pseudomallei was enhanced by increasing the concentrations of iron significantly (P < 0.05). We propose using these findings to control B. pseudomallei in situ.

Figure 1.

Effect of pH and incubation time on the survival of B. pseudomallei (log CFU/mL) in the soil microcosm at pH 4–10. CFUs per milliliter were determined as the colony counts on the modified Ashdown's agar using the dropped plate method at 37°C for 7 days. Points show means (N = 6); vertical lines denote SD.

Figure 2.

Effect of NaCl and incubation time on the survival of B. pseudomallei (log CFU/mL) in the soil microcosm at soil salinity 0.0% to 5.0% NaCl. CFUs per milliliter were determined as the colony counts on the modified Ashdown's agar using the dropped plate method at 37°C for 7 days. Points show means (N = 6); vertical lines denote SD.

Figure 3.

Effect of Fe and incubation time on the survival of B. pseudomallei (log CFU/mL) in the soil microcosm at total iron of 50–150 mg/kg. CFUs per milliliter were determined as the colony counts on the modified Ashdown's agar using the dropped plate method at 37°C for 7 days. Points show means (N = 6); vertical lines denote SD.

Figure 4.

Effect of C/N ratio and incubation time on the survival of B. pseudomallei (log CFU/mL) in the soil microcosm at C/N ratios from 10:1 to 40:1. CFUs per milliliter were determined as the colony counts on the modified Ashdown's agar using the dropped plate method at 37°C for 7 days. Points show means (N = 6); vertical lines denote SD.