Retention of water-borne bacteria by membrane filters. Part II: Scanning electron microscopy (SEM) and fatty acid methyl ester (FAME) characterization of bacterial species recovered downstream of 0.2/0.22 micron rated filters

Abstract

The results of scanning electron microscopic (SEM) and fatty acid methyl ester (FAME) characterization of the bacterial species shown to penetrate conventional 0.2/0.22 micron rated "sterilizing grade" filters are presented. SEM data suggest that retention of bacteria by these filters appears to be strongly influenced by the morphology, and especially the width of bacteria and less so by length. When the bacterial cell width is small, less than 0.3 micron or so, the cell length does not appear to limit the ability to penetrate 0.2/0.22 micron rated filters. As the bacterial width increases, there is also a strong, almost exponential, decrease in the allowable length for penetration, with most penetrative cells tending to be coccoid beyond a width of 0.5 micron. Significant percentages of the bacteria (40-50%) that were observed downstream of these filters were larger than B. diminuta, the standard organism used to qualify 0.2/0.22 micron rated filters. The average sizes of natural waterborne bacteria that penetrated the filters tested were 20-40% larger in width, and 40-70% larger in length, compared to B. diminuta. These results indicate that size exclusion is not the sole mechanism governing bacterial retention. All isolates identified via FAME analyses were common environmental or ubiquitous organisms, and some, such as Acidovorax sp. and Hydrogenophaga pseudoflava, have also been isolated from pharmaceutical water systems. Most of the bacteria recovered downstream of 0.2/0.22 micron rated filters were gram negative, oxidase positive, motile, nonfermentors.