Modelling the thermal inactivation of Salmonella typhimurium using bioluminescence data

Predicting Salmonella Typhimurium reductions in poultry ground carcasses

To improve understanding of Salmonella Typhimurium LT2 inactivation in ground poultry carcasses, a series of experiments were carried out at multiple temperatures. Subsequently, a non-linear model was developed to predict Salmonella inactivation at composting and low rendering temperatures. The Salmonella inactivation study was conducted using bench-top experiments at 38, 48, 55, 62.5, 70, and 78°C in mixed and non-mixed reactors using ground poultry carcasses as a feedstock. Subsequently, these observations were used for developing a non-linear model. The model predictions were compared with the observations of a different set of experiments. The comparisons among predictions and observations showed that the model predictions are reasonable and can be useful to determine the time required for Salmonella inactivation in poultry carcasses at multiple temperatures. Results showed that at composting conditions, when temperature varies between 48 and 62.5°C, Salmonella survival can prolong between 10,000 and 25,000 min (7 to 17 d). If ambient temperature is maintained at low temperature rendering range (70 to 78°C), then Salmonella survival can last for 90 to 120 minutes. We anticipate that this study will help in improving the existing understanding of Salmonella survival in poultry carcasses.

Pressure inactivation kinetics of Enterobacter sakazakii in infant formula milk

Survival curves of Enterobacter sakazakii inactivated by high hydrostatic pressure were obtained at four pressure levels (250, 300, 350, and 400 MPa), at temperatures below 30 degrees C, in buffered peptone water (BPW; 0.3%, wt/vol) and infant formula milk (IFM; 16%, wt/vol). A linear model and four nonlinear models (Weibull, log-logistic, modified Gompertz, and Baranyi) were fitted to the data, and the performances of the models were compared. The linear regression model for the survival curves in BPW and IFM at 250 MPa has fitted regression coefficient (R2) values of 0.940 to 0.700, respectively, and root mean square errors (RMSEs) of 0.770 to 0.370. For the other pressure levels, the linear regression function was not appropriate, as there was a strong curvature in the plotted data. The nonlinear regression models with the log-logistic and modified Gompertz equations had R2 values of 0.960 to 0.992 and RMSE values of 0.020 to 0.130 within pressure levels of 250 to 400 MPa, respectively. These results indicate that they are both better models for describing the pressure inactivation kinetics of E. sakazakii in IFM and BPW than the Weibull distribution function, which has an R2 minimum value of 0.832 and an RMSE maximum value of 0.650 at 400 MPa. On the other hand, the parameters for the Weibull distribution function, log-logistic model, and modified Gompertz equation did not have a clear dependence on pressure. The Baranyi model was also analyzed, and it was concluded that this model provided a reasonably good fit and could be used to develop predictions of survival data at pressures other than the experimental pressure levels in the range studied. The results provide accurate predictions of survival curves at different pressure levels and will be beneficial to the food industry in selecting optimum combinations of pressure and time to obtain desired target levels of E. sakazakii inactivation in IFM.

Use of linear, Weibull, and log-logistic functions to model pressure inactivation of seven foodborne pathogens in milk

Survival curves of six foodborne pathogens suspended in ultra high-temperature (UHT) whole milk and exposed to high hydrostatic pressure at 21.5 degrees C were obtained. Vibrio parahaemolyticus was treated at 300 MPa and other pathogens, Listeria monocytogenes, Escherichia coli O157:H7, Salmonella enterica serovar Enteritidis, Salmonella enterica serovar Typhimurium, and Staphylococcus aureus were treated at 600 MPa. All the survival curves showed a rapid initial drop in bacterial counts followed by tailing caused by a diminishing inactivation rate. A linear model and two nonlinear models were fitted to these data and the performances of these models were compared using mean square error (MSE) values. The log-logistic and Weibull models consistently produced better fits to the inactivation data than the linear model. The mean MSE value of the linear model was 6.1, while the mean MSE values were 0.7 for the Weibull model and 0.3 for the log-logistic model. There was no correlation between pressure resistance and the taxonomic group the bacteria belong to. The order, most to least pressure-sensitive, of the single strains tested was: V. parahaemolyticus (gram negative)<L. monocytogenes (gram positive)<Salmonella Typhimurium (gram negative) approximately = Salmonella Enteritidis (gram negative)<E. coli O157:H7 approximately = Staphylocollus aureus (gram positive)<Shigella flexneri (gram negative). The most pressure-resistant gram-negative bacterium, Shigella flexneri, and most pressure resistant gram-positive bacterium, Staphylocollus aureus, were pressurized at 50 degrees C. Staphylocollus aureus was treated at 500 MPa and Shigella flexneri at 600 MPa. Elevated temperature considerably enhanced pressure inactivation of these two pathogens, but did not affect the overall shape of the survival curves. Pressure level (250 MPa) and substrate (1% peptone water plus 3% NaCl) in which V. parahaemolyticus was suspended affected the shape of survival curves of V. parahaemolyticus.

Effect of challenge temperature and solute type on heat tolerance of Salmonella serovars at low water activity

Salmonella spp. are reported to have an increased heat tolerance at low water activity (a(w); measured by relative vapor pressure [rvp]), achieved either by drying or by incorporating solutes. Much of the published data, however, cover only a narrow treatment range and have been analyzed by assuming first-order death kinetics. In this study, the death of Salmonella enterica serovar Typhimurium DT104 when exposed to 54 combinations of temperature (55 to 80 degrees C) and a(w) (rvp 0.65 to 0.90, reduced using glucose-fructose) was investigated. The Weibull model (LogS = -bt(n)) was used to describe microbial inactivation, and surface response models were developed to predict death rates for serovar Typhimurium at all points within the design surface. The models were evaluated with data generated by using six different Salmonella strains in place of serovar Typhimurium DT104 strain 30, two different solutes in place of glucose-fructose to reduce a(w), or six low-a(w) foods artificially contaminated with Salmonella in place of the sugar broths. The data demonstrate that, at temperatures of > or =70 degrees C, Salmonella cells at low a(w) were more heat tolerant than those at a higher a(w) but below 65 degrees C the reverse was true. The same patterns were generated when sucrose (rvp 0.80 compared with 0.90) or NaCl (0.75 compared with 0.90) was used to reduce a(w), but the extent of the protection afforded varied with solute type. The predictions of thermal death rates in the low-a(w) foods were usually fail-safe, but the few exceptions highlight the importance of validating models with specific foods that may have additional factors affecting survival.

Review of studies on the thermal resistance of Salmonellae

Heat resistance data for different serotypes of Salmonella enterica in different food products and laboratory media are reviewed. From all D-values reported, the highest heat resistance of Salmonella was in liquid eggs and liquid egg yolks. The equation from a line drawn through the highest D-values, and above all values reported, was log D-value = 11.7 - 0.188T degrees C. From this equation, the calculated z-value was 5.3 degrees C (9.5 degrees F), and a process at 71degrees C (160 degrees F) will require 1.2 s to inactivate 1 log of Salmonella cells. This calculation did not include data that evaluated the heat resistance after stress conditions or data for Salmonella Senftenberg. The heat resistance of Salmonella is highly influenced by the strain tested, the type of experiment (log reduction versus end-point), culture conditions prior to the experiment, heating menstruum, and recovery conditions. Heat resistance data for Salmonella are still nonexistent or scarce in chicken meat, fruit juices, and aquacultured fish.

How novel methods can help discover more information about foodborne pathogens

Considerable emphasis is being placed on quantitative risk assessment modelling as a basis for regulation of trade in food products. However, for models to be accurate, information about the behaviour of potential pathogens in foods needs to be available. The question is how to obtain this knowledge in a simple and cost effective way. One technique that has great potential is the use of reporter bacteria which have been genetically modified to express a phenotype that can be easily monitored, such as light production in luminescent organisms. Bacteria carrying these (lux) genes can easily be detected using simple luminometers or more sophisticated low light imaging equipment.By monitoring light output from these bacteria over time, it can easily be determined if the organism is growing (resulting in an increase in light emission), is dead (causing a decrease in light production) or is injured (light output remains constant). The use of imaging systems allows the response of bioluminescent bacteria to be studied directly on the food, making the technique even more useful. Applications of bioluminescence are discussed below and include use as reporters of gene expression; biocide efficacy and antibiotic susceptibility; sub-lethal injury; adhesion and biofilm formation; the microbial ecology of foods; pathogenesis; and as biosensors.

Application of thin agar layer method for recovery of injured Salmonella typhimurium

Xylose lysine decarboxylase (XLD) medium, a selective plating medium, can inhibit heat-injured Salmonella typhimurium from growing, whereas tryptic soy agar (TSA), a nonselective medium, does not. To facilitate recovery of heat-injured S. typhimurium cells while providing selectivity of isolation of S. typhimurium from other bacteria in the sample, a thin agar layer (TAL) procedure was developed by overlaying 14 ml of nonselective medium (TSA) onto prepoured and solidified XLD medium in a 8.5 cm diameter Petri dish. During the first few hours of incubating the plate, the injured S. typhimurium repaired and started to grow in the TSA. During the resuscitation of injured cells, the selective agents from XLD were diffused to the TSA top layer part. Once the selective agents diffused to the top part of the TAL, the resuscitated S. typhimurium started to produce a typical reaction (black color) and other microorganisms were inhibited by the selective agents. The recovery rate for heat-injured (55 degrees C for 15 min) S. typhimurium with the TAL method was compared with TSA, XLD, and the traditional overlay method (OV; pouring selective agar on top of resuscitated cells on TSA agar 3-4 h after incubation). No significant difference occurred among TSA, OV, and TAL (P > 0.05) for enumeration of heat-injured S. typhimurium, but they recovered significantly higher numbers than from XLD agar (P < 0.05).

Real-time monitoring of Escherichia coli O157:H7 adherence to beef carcass surface tissues with a bioluminescent reporter

A method for studying bacteria that are attached to carcass surfaces would eliminate the need for exogenous sampling and would facilitate understanding the interaction of potential human food-borne pathogens with food animal tissue surfaces. We describe such a method in which we used a bioluminescent reporter strain of Escherichia coli O157:H7 that was constructed by transformation with plasmid pCGLS1, an expression vector that contains a complete bacterial luciferase (lux) operon. Beef carcass surface tissues were inoculated with the bioluminescent strain, and adherent bacteria were visualized in real time by using a sensitive photon-counting camera to obtain in situ images. The reporter strain was found to luminesce from the tissue surfaces whether it was inoculated as a suspension in buffer or as a suspension in a bovine fecal slurry. With this method, areas of tissues inoculated with the reporter strain could be studied without obtaining, excising, homogenizing, and culturing multiple samples from the tissue surface. Use of the complete lux operon as the bioluminescent reporter eliminated the need to add exogenous substrate. This allowed detection and quantitation of bacterial inocula and rapid evaluation of adherence of a potential human pathogen to tissue surfaces. Following simple water rinses of inoculated carcass tissues, the attachment duration varied with different carcass surface types. On average, the percent retention of bioluminescent signal from the reporter strain was higher on lean fascia-covered tissue (54%) than on adipose fascia-covered tissue (18%) following water washing of the tissues. Bioluminescence and culture-derived viable bacterial counts were highly correlated (r2 = 0.98). Real-time assessment of microbial attachment to this complex menstruum should facilitate evaluation of carcass decontamination procedures and mechanistic studies of microbial contamination of beef carcass tissues.

A mathematical model for bacterial inactivation

The first order kinetic model, the Buchanan model and Cerf's model, can model a linear survival curve, a survival curve with a shoulder and a survival curve with a tailing, respectively. However, they are not suitable for fitting a sigmoidal survival curve. The three models were integrated into a new model that was capable of fitting the four most commonly observed survival curves: linear curves, curves with a shoulder, curves with a tailing (biphasic curves) and sigmoidal curves. The new model was compared with the Whiting-Buchanan model using the survival curves of Staphylococcus aureus. The goodness-of-fit of the proposed model is practically as good as that of the Whiting-Buchanan model. Compared with the Whiting-Buchanan model, the proposed model has a more mechanistic background. Since for non-linear survival curves, such as biphasic and sigmoidal curves, the t(m-D) value (the time required for an m-log-cycle reduction of microorganisms under a given condition) cannot be estimated accurately by the existing or traditional method, a new method is also proposed to predict accurately the t(m-D) value for non-linear survival curves.

The application of a log-logistic model to describe the thermal inactivation of Clostridium botulinum 213B at temperatures below 121.1 degrees C

In this work, the death of Clostridium botulinum 213B was measured at temperatures between 101 degrees C and 121 degrees C. It was found that at all temperatures tested, survivor curves deviated from log-linearity which prevented their description using traditional first order kinetics. The survivor curves were better described using a vitalistic approach and the log-logistic transformation proposed by Cole et al. (1993). A single equation was derived to describe all survivor curves over the temperature range tested and a comparison of predicted and measured data showed good correlation. The implications of the use of the vitalistic approach to the validity of the 'minimum botulinum cook' is discussed.

Use of bioluminescence to model the thermal inactivation of Salmonella typhimurium in the presence of a competitive microflora

The survival of Salmonella typhimurium was investigated by bioluminescence and standard plating techniques in pure cultures and in the presence of competitors after the cultures were heated to 55 degrees C for increasing lengths of time. Decimal reduction (D) values increased from 0.43 to 2.09 min in the presence of 10(8) CFU of competitors ml-1, indicating a significant protective effect.

Susceptibility of suspended and surface-attached Salmonella enteritidis to biocides and elevated temperatures

The differential resistance of substratum-attached, detached, and planktonic cells of Salmonella enteritidis phage type 4 was studied by using several inimical processes and in vivo bioluminescence as a nondestructive, real-time reporter of metabolic activity. Bioluminescence in this strain was mediated by a construction containing the entire lux operon from Photorhabdus luminescens. An excellent correlation between bioluminescence and classical plate count data was obtained when we compared attachment profiles, biocide concentration exponents, and thermal inactivation D values (D value was the time required for a 10-fold reduction in the number of survivors). Biocide challenge of surface-adherent S. enteritidis resulted in concentration exponents that were experimentally indistinguishable from those obtained with Luria-Bertani broth-grown planktonic cells. It appears that cleansing regimes developed by using planktonic cell data are effective against surface-attached cells of this bacterium. Both attached and detached cells exhibited an approximately twofold increase in D values at 52 degrees C compared with values calculated for planktonic cells, strongly indicating that the detached cells exhibited an attached phenotype during the heating process. A model of a physiological adaptive response induced in attached cells and also reflected in detached cells is presented.