Passage of bacterial cells through filter sizes has been extensively studied especially in relation to water treatment and pharmaceutical applications, and several explanations given for the observed passage even under the most unlikely circumstances. Oppenheimer and others who have studied filterability of microorganisms from natural mineral water attributed this passage to the possible presence of smaller bacterial cells often referred to as ultra-microcells [5, 6]. Grinnell's results showed that culture age affected filterability with older cultures being capable of rapid passage through filter pores than fresher cultures . Amberg concluded from his studies that passage through filters was time-dependent with filterable forms detectable only after the initial 10 min interval of filtration . Wang and colleagues concluded from their filtration studies that the overall shape and flexibility of a bacterium affected its passage through small pore sizes .
In this present study, we tested the filterability of bacterial cultures through industrial filter pores following application of a range of stress conditions. We observed passage for S. aureus, S. epidermidis and S. lugdunensis through the filter pores while both E. coli and B. cereus were inhibited suggesting a species-specific advantage and/or shape advantage. Application of a stress condition did not appear to increase filterability of the cultures, since in most treatment cases, the numbers of cells passing through were fewer in comparison to passage of control samples. There was more passage of bacterial cells through 0.45 μm filters than through the 0.22 μm filters, suggesting a wide range of cell sizes were present in these cultures. Passage of staphylococcal cells through the 0.22 μm filter was mostly inhibited suggesting that only a very small proportion of cells were small enough to pass through. Therefore, this pore size was a better choice for sterilization where staphylococci are involved. Although passage through this pore size was observed for fresh, old and temperature stressed cultures, the occurrences were relatively low. A previous study reported that some 0.2 μm filters consisted of a distribution of pore sizes with some at least as big as 0.5 μm in size  and although possible, is highly unlikely the reason for the observed result in our study.
From our results, both overall shape and size of the bacterium seemed to be contributing factors to the observed filterability. B. cereus and E. coli are both rod-shaped bacteria while the staphylococci are spherical shaped, suggesting that the latter's shape was more suitable to penetrating through small pores. Moreover, Staphylococci have a reported size range between 0.9-1.3 μm while the Bacillus and E. coli are 1.2-10 μm long and 0.5-2.5 μm wide . Their larger dimensions could be plausible reasons why their passage through the filter pores was not observed. Nonetheless, passage of Bacillus and E. coli through 0.2 μm membranes has been reported, although their passage was also found to be dependent on other factors (time and volume of inoculum)  not tested here.
Reports have shown that cultures under stress (eg. lack of nutrient availability) have smaller cells than those growing under optimal conditions [13, 20]. With this being true, then it would be expected that other forms of stress would consequently have the same effect on cell-size with smaller cells which would then be capable of passing with greater ease through small pore sizes. Exponentially-growing cultures were exposed to a stressor and incubated for an additional 5 hrs to allow for possible "small forms" to develop. Unfiltered stressed samples when sub-cultured showed a reduction in colony size 24 hrs post-incubation when compared to control samples, indicating that the applied stress had indeed affected cell growth and/or division, which was reflected in colony size. Filtered samples showed an increase in filterability for some S. aureus treatment samples compared to control samples of the same but most treatments showed reduced passage of cells. In contrast, both the coagulase negative staphylococci (CNS) S. epidermidis and S. lugdunensis showed a reduced filterability in all instances when the stress factors were introduced. It is possible that the stressors affected the viability of the cells in the population thereby reducing the proportion of viable cells passing through the filters. This was evident when CFU/mL were significantly fewer for non-filtered stressed samples compared to their corresponding control samples. Thus, even though smaller cells may have existed in the population as suggested by Grinnell , their numbers may have been relatively low.
One of the more interesting outcomes of this investigation was the greater filterability of S. lugdunensis cultures. This species was better able to pass through at relatively higher numbers, regardless of the treatment condition, while both S. aureus and S. epidermidis showed limited passage or complete inhibition. This observed ability of S. lugdunensis to successfully penetrate both 0.45 μm and 0.22 μm sterile filters and grow normal colonies upon filtrate sub-culture, presents concern not only for applications that use micro-filtration as the sole means of sterilization where S. lugdunensis is involved, but physiological consequences in disease pathogenesis with this bacterium. S. lugdunensis is commonly isolated from and implicated as the cause of several nosocomial infections  and the ability of this opportunistic pathogen to pass through into sterile solutions and remain viable poses potential problems for clinical microbiology. Wainwright  suggests that the ability of such bacteria to pass through small holes could pose serious pathological consequences in instances where penetration of membranes, host cells and tissues is possible.