Filterability of staphylococcal species through membrane filters following application of stressors

Results showed repeated passage of viable staphylococcal cells through both pore sizes, although more passage was seen through the 0.45 μm pore size. Of the three staphylococcal species, S. lugdunensis showed the best passage at relatively higher numbers regardless of the treatment, while both S. aureus and S. epidermidis showed limited passage or complete inhibition.The data showed that staphylococcal bacteria were capable of passing through sterile filters in a viable state. There was better passage through 0.45 μm sterile filters than through the 0.22 μm sterile filters. Application of a stress condition did not appear to enhance filterability of these bacterial cultures.Microfiltration processes that utilise membrane filters to sterilise liquids have been in use since the early 20th century. They are useful for the removal of contaminants and major pathogens in applications where sterile products are required. Over the years, these filters have been developed further to suit their purpose with pore sizes of 0.2 μm or less, regarded as effective against retention of microbial entities that compromise the sterility of products. Nonetheless, passage of bacterial cells through membrane filters has been repeatedly observed [1-4], even though the mechanisms of cell passage through even the smallest pore sizes is sometimes unknown. Studies conducted on mineral water samples have suggested the presence of bacterial cells as small as 0.15 μm in diameter allowing for the observed passage. Oppenheimer coined these cells ultra-microcells [5]. Although the study of filterable forms was prominent in the early 20th century, interest in this area waned through the years, but has recently been re-ignited with the introduction of the concept of nanobacteria, a class of exceptionally small cells (80-200 nm in diameter) [6]. Although the presence of nanobacteria has been suggested in geology [7] and disease pathology [8,9], their existence and properties are heavily debated concept