Scott K. Hood’s research while affiliated with St. Catherine University and other places

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the means by which three strains of Lactobacillus acidophilus adhere to surfaces. When observed with SEM, all strains adhered to glass coverslips; however, no attachment fibrils were seen. When viewed with TEM, strain BG2F04 showed a polysaccharide (PS) material outside the cell wall. This strain had previously been shown to adhere strongly to human intestinal tissue cells, while strain Lac 12 had shown weak adherence, and strain NCFM did not adhere. The PS layer on strain Lac 12 was variable, and a PS layer on NCFM could not be seen. These observations suggested that this PS material may be involved in the adherence of strain BG2F04 to surfaces.

Lactobacillus acidophilus was suspended in broth and buffer at pH 2, 3 and 4 and incubated at 37°C for 2 hr. In both broth and buffer pH 2, viable cell numbers decreased rapidly, and none were recovered after 45 min. At pH 4 in broth and buffer, the number of cells was not significantly reduced in 2 hr. In potassium phthalate buffer at p 3, no viable cells were recovered after 30 min, while in KCl buff and broth at pH 3, no significant reduction was seen. Cells subjected to low pH for up to 5 h were able to adhere to human intestinal cells in vitro. Exposure to low pH did not appear to disrupt the ruthenium red staining layer exterior to the cell wall.

Biofilms are described as a matrix of microorganisms which have adhered to and colonized a surface. Once formed, biofilms are difficult to remove and may be a source of contamination in food-processing environments. In this study, stainless-steel chips were fixed to surfaces adjacent to food-contact surfaces and cast-iron chips were suspended in the floor drains of four meat-processing plants. Biofilm formation was quantified by staining the attached cells and viewing them under epifluorescence microscopy. The stainless-steel and cast-iron chips removed from the plant environment showed some attached microorganisms. Floor drains appeared to provide an excellent environment for the formation of biofilms. Pseudomonas, Klebsiella, Aeromonas, and Hafnia species were identified as gram-negative microorganisms associated with the test surfaces.

Microorganisms have been shown to adhere to food-contact surfaces and may provide a route for the contamination of processed food. To better understand this phenomenon, the effects of growth media and surface conditioning on the adherence of Pseudomonas fragi, Salmonella typhimurium and Listeria monocytogenes cells to stainless steel were studied. The microorganisms were grown in tryptic soy broth (TSB), 1% reconstituted skim milk (RSM) and RSM with 1% sucrose (RSM + S). Stainless-steel surfaces were conditioned by immersion in growth media for 1 h and then were rinsed in phosphate-buffered saline (PBS) prior to the adherence assay. After growing in each medium, cells were harvested, resuspended in PBS, and then allowed to contact the stainless steel for 30 min. Adherence was quantified by acridine orange-staining the cells and viewing under epifluorescence microscopy. Growth media had little influence on adherence to stainless steel that had not been preconditioned. P. fragi and L. monocytogenes cells adhered in the highest numbers when grown in RSM plus sucrose. S. typhimurium cells showed the highest level of adherence when grown in TSB. Analysis of variance yielded P values of less than 0.01, indicating that both growth media and surface conditioning were significant in the level of adherence observed.

Biofilm formation on stainless steel by Salmonella typhimurium, Listeria monocytogenes, Escherichia coli O157:H7, Pseudomonas fragi and Pseudomonas fluorescens during growth in model food systems was studied. Test growth media included tryptic soy broth (TSB), diluted TSB (dTSB), 1% reconstituted skim milk (RSM) and diluted meat juice (DMJ). Adherent cells were stained with acridine orange and enumerated using epifluorescent microscopy and computerized image analysis. Cells were observed on the stainless steel surface after 1 h in all of the media. However, the increases in the number of adherent cells over time was seen only with S. typhimurium in DMJ, E. coli O157:H7 in TSB, dTSB and DMJ, P. fragi in RSM and P. fluorescens in RSM. The medium which produced the highest observed level of adherent cells was different for each microorganism.

Microbial colonization of surfaces (biofilms) have been documented in many environments. Recently, researchers have suggested that biofilms may be a source of contamination in food processing environments. This review will discuss some historical aspects of biofilms, possible mechanisms for the adherence of bacteria to surfaces, methods for studying biofilms and problems adherent microorganisms may cause in food processing.