Lillis T O, Bissonnette G K
Division of Plant and Soil Sciences, West Virginia University, Morgantown 26506-6057, USA.
Lett Appl Microbiol. 2001 Apr;32(4):268-72. doi: 10.1046/j.1472-765x.2001.00902.x.
The chemical/physical environment of groundwater may contribute to the existence of a subpopulation of small-sized bacteria (filterable bacteria) that fails to be trapped on conventional 0.45 microm-pore-size membrane filters during routine bacteriological water quality analyses. Efforts were directed to determining an efficient recovery method for detection of such cells.
Individual groundwater supplies in a rural setting were examined by a double membrane filtration procedure to determine the presence of heterotrophic plate count (HPC) bacteria capable of escaping detection on conventional pore size (0.45 microm) membrane filters but retained on 0.22 microm-pore-size filters. Since optimum cultural conditions for recovery of filterable bacteria are not well defined, initial efforts focused on evaluation of various media (R2A, m-HPC and NWRI) and incubation temperatures (15, 20, 28 and 35 degrees C) for specific recovery of filterable bacteria. Maximum recovery of small-sized HPC bacteria occurred on low-nutrient concentration R2A agar incubated for 7 d at 28 degrees C. Similarly, identical cultural conditions gave enhanced detection of the general HPC population on 0.45 microm-pore-size filters. A 17-month survey of 10 well water supplies conducted with the cultural conditions described above resulted in detection of filterable bacteria (ranging in density from 9 to 175 cfu ml-1) in six of the groundwater sources. The proportion of filterable bacteria in any single sample never exceeded 10% of the total HPC population. A majority of the colonies appearing on the 0.22 microm membrane filters was pigmented (50-90%), whereas the proportion of colonies demonstrating pigmentation on the larger porosity filters failed to exceed 50% for any of the samples (19-49%).
A reliable recovery method was developed for the detection of filterable bacteria from groundwater. During a subsequent survey study using this procedure, filterable bacteria were detected in a majority of the groundwater supplies examined; however, the density of filterable bacteria in any single sample never exceeded 10% of the total HPC population. Identification of randomly selected isolates obtained on the 0.22 microm filters indicated that some of these filterable bacteria have been implicated as opportunistic pathogens.
We have determined the presence of small-sized HPC bacteria in ground water that may go undetected when using standard porosity membrane filters for water quality analyses. Further study is needed to assess the significance and possible health risk associated with presence of filterable bacteria in drinking water supplies from groundwater sources.
地下水的化学/物理环境可能导致存在一小部分小型细菌（可滤过细菌），在常规水质细菌学分析过程中，这些细菌无法被截留在传统的0.45微米孔径的膜滤器上。研究致力于确定一种有效的回收方法来检测这类细胞。
通过双膜过滤程序对农村地区的各个地下水供应源进行检测，以确定是否存在能够逃避传统孔径（0.45微米）膜滤器检测但能被0.22微米孔径滤器截留的异养平板计数（HPC）细菌。由于可滤过细菌回收的最佳培养条件尚不明确，最初的研究重点是评估各种培养基（R2A、m - HPC和NWRI）以及培养温度（15、20、28和35摄氏度）对可滤过细菌的特异性回收效果。在28摄氏度下于低营养浓度的R2A琼脂上培养7天，小型HPC细菌的回收率最高。同样，相同的培养条件也能增强在0.45微米孔径滤器上对一般HPC菌群的检测。采用上述培养条件对10个井水供应源进行了为期17个月的调查，结果在6个地下水源中检测到了可滤过细菌（密度范围为9至175 cfu/ml）。任何单个样本中可滤过细菌所占比例从未超过总HPC菌群的10%。在0.22微米膜滤器上出现的大多数菌落都带有色素（50 - 90%），而在孔隙率较大的滤器上出现色素的菌落比例，对于任何样本都未超过50%（19 - 49%）。
开发了一种可靠的回收方法用于检测地下水中的可滤过细菌。在随后使用该程序的调查研究中，在所检测的大多数地下水供应源中都检测到了可滤过细菌；然而，任何单个样本中可滤过细菌的密度从未超过总HPC菌群的10%。对在0.22微米滤器上随机选取菌株的鉴定表明，其中一些可滤过细菌被认为是机会致病菌。
我们已确定地下水中存在小型HPC细菌，当使用标准孔隙率膜滤器进行水质分析时，这些细菌可能未被检测到。需要进一步研究以评估与地下水源饮用水供应中可滤过细菌的存在相关的意义和可能的健康风险。
