Mercury decreases culturability of Pseudomonas frederiksbergensis JAJ 28 in soil microcosms

Mercury is a biologically potent heavy metal, which has been found to change the diversity of culturable bacteria. Therefore, we investigated whether Hg kills bacteria in soil or reduces culturability. Soil microcosms were inoculated with Pseudomonas frederiksbergensis JAJ 28 and were sampled regularly during 28 days. The total number of acridine orange-stained cells was relatively constant, and Hg reduced the number on only one sampling day. However, the fraction of culturable cells on 1/10 tryptic soy agar was lowered on days 6, 13, and 21. The number of microcolony forming units, which represents viable cells, was also affected by Hg, but this effect was delayed compared with the effects on CFUs. The amount of headspace CO2 per cell was overall increased by Hg, another indication of the toxic effects of Hg on the bacterial cells. Our results thus emphasize the need to take culturability into account when studying the effects of heavy metals on bacterial diversity.