The Detection of Microorganisms and Organic Material on Stainless Steel Food Contact Surfaces

Few methods are available for the differentiation of microorganism and organic material on surfaces, although such mixtures are commonplace, particularly in the food industry, where food debris (soil) and microorganisms frequently foul food contact surfaces and pose challenges in terms of hygiene and cleanability. It would be of value to discern any differences in removal or persistence on surfaces. This review considers some methods which are available. Direct epifluorescence microscopy (DEM) enables visual differentiation, but traditional microbiological culture methods cannot detect organic soil. Atomic force microscopy (AFM) provides images of the surface on the nanometer scale, with minimal preparation, and is able to visualise both cellular and acellular components of the mixture, particularly prior to cleaning. Confocal laser scanning microscopy (CLSM) also has potential in this area. Surface sensitive methods such as X-Ray photoelectron spectroscopy (XPS) and Time-of-flight secondary ion mass spectrometry (ToFSIMS) provide information on chemical species present on a surface. Those chemical species more likely on microbial cells may be differentiated from those more likely in a specified organic soil, thus comparisons may be made as to differential removal of the organic soil and microbial cells. These methods may be of value in studies on the fouling and cleanability of surfaces.     

The detection of food soils on stainless steel using energy dispersive X-ray and Fourier transform infrared spectroscopy

Organic soiling is a major issue in the food processing industries, causing a range of biofouling and microbiological problems. Energy dispersive X-ray (EDX) and Fourier transform infra red spectroscopy (FT-IR) were used to quantify and determine the biochemical groups of food soils on stainless steel surfaces. EDX quantified organic material on surfaces where oily based residues predominated, but was limited in its usefulness since other food soils were difficult to detect. FT-IR provided spectral ‘fingerprints’ for each of the soils tested. Key soiling components were associated with specific peaks, viz. oils at 3025 cm^?1–3011 cm^?1, proteins at 1698 cm^?1–1636 cm^?1 and carbohydrates at 1658 cm^?1–1596 cm^?1, 783 cm^?1–742 cm^?1. High concentrations of some soils (10%) were needed for detection by both EDX and FT-IR. The two techniques may be of use for quantifying and identifying specific recalcitrant soils on surfaces to improve cleaning and hygiene regimes.     

The retention of bacteria on hygienic surfaces presenting scratches of microbial dimensions

Aims: To produce surfaces of defined linear topographical features which reflect those found on worn and new stainless steel, to monitor the effect of feature dimensions on the retention of Listeria monocytogenes and Staphylococcus sciuri. Methods and Results: Surfaces were fabricated with parallel linear features of 30 microns or of microbial dimensions (1·02 and 0·59 μm width) and used in microbial retention assays with Staph. sciuri and L. monocytogenes. Retained cells were distributed uniformly across the smooth 30 micron featured surfaces but were retained in high numbers on microtopographies at the ‘peaks’ between the wide grooves. On smaller features, retention was attributed to the maximum area of contact between cells and substratum being attained, with cocci being embedded in 1·02‐μm‐width grooves, and rods aligned along (and across) the densely packed parallel 0·59‐μm grooves. Conclusions: The dimensions of surface features may enhance or impede cell retention. This phenomenon is also related to the size and shape of the microbial cell. Significance and Impact of Study: Findings may help describe and evaluate properties of hygienic and easily cleanable surfaces.     

The survival and growth of an environmental Klebsiella isolate in detergent solutions

A Klebsiella isolate (G1) was capable of survival and growth in an environment containing high levels of anionic (15.6%) and non-ionic detergent (7.8%). Cell numbers were monitored by traditional viable plate counts (culturability), and by staining with Rhodamine 123 (vital counts) and Acridine Orange (total counts). On inoculation into detergent solutions, only 10% of cells retained vitality (rhodamine 123) after 24 h. Of those, only 1% were able to form colonies on artificial culture media. Under low nutrient conditions (TOC < 0.89 mg l - 1), no recovery was observed over a 96 h period, but addition of nutrients (TOC 3.84 mg l - 1) allowed recovery within 48 h. Such cells were initially (24 h) elongated, but subsequently (144 h) returned to their normal size. In the presence of detergent without added nutrient, cell size was reduced after 144 h. After adaptation to the detergent environment (24 - 48 h), cell numbers in detergent with added nutrient broth (vital, viable and total) rose, until levels equivalent to those of a detergent-free control were attained. Detergent solutions provide a stressful environment. Nutrient levels in the detergent solutions affected cell size and the ability of the Klebsiella (G1) to survive and grow.     

Culture media for differential isolation of Lactobacillus casei Shirota from oral samples

This study aimed to develop a solid culture medium for differential isolation of the probiotic strain Lactobacillus casei Shirota (LcS) and for selective cultivation of lactobacilli present in oral samples. Type strains of lactobacilli and isolates from commercial probiotic products were inoculated onto modified de Man Rogosa Sharpe agar (termed 'LcS Select'), containing bromophenol blue pH indicator, vancomycin and reducing agent L-cysteine hydrochloride for differential colony morphology development. L. casei Shirota cultured on the novel medium produced distinctive colony morphologies, different from other lactobacilli tested. LcS-characteristic colonies were recovered on LcS Select medium from samples of saliva and tongue plaque following a four-week probiotic intervention study. The viable count of presumptive LcS colonies correlated with those isolated on a non-commercial lactitol-LBS-vancomycin agar (LLV) developed for a selective isolation of LcS from faeces. The novel LcS Select medium proved suitable for differential isolation of the probiotic strain L. casei Shirota from oral samples containing mixed microbial populations. It can also be used for selective growth of vancomycin-resistant lactobacilli. There are few available culture media that are sufficiently selective to enable isolation of probiotic strains from mixed populations. LcS Select medium provides a cheaper, yet effective tool in this context.     

Refreshing the public appetite for ‘good bacteria’: menus made by microbes

A series of events was developed to engage audiences in discussion around the importance of microorganisms in the production of fermented foods, particularly through tasting. The events were designed to attract different audiences: families attending a science museum; families in their community space; and adults in different informal eating environments. Information was provided, alongside interactive activities where possible. Feedback was positive in that the audiences enjoyed the format, the food and the events themselves, although science itself was not always specifically mentioned. The dining experience format provided a versatile and informal opportunity for engagement between experts and non-experts, and is suggested as a valuable template for similar activities, assuming appropriate budgeting and advertising issues are addressed, alongside appropriate objective setting and effective evaluation.