Variability in lag-duration of Campylobacter spp. during enrichment after cold and oxidative stress and its impact on growth kinetics and reliable detection

Comparison of optical density-based growth kinetics for pure culture Campylobacter jejuni, coli and lari grown in blood-free Bolton broth

Campylobacter growth kinetic parameters can be used to refine the sensitivity and efficiency of microbial growth-based methods. Therefore, the aim of this study was to construct growth curves for C. jejuni, C. coli, and C. lari in pure culture and calculate growth kinetics for each Campylobacter species in the same environmental conditions. Campylobacter jejuni, C. coli and C. lari were grown over 48 h and inoculated into 15 mL Hungate tubes (N = 3 trials per species; 5 biological replicates per trial; 3 species; 1 strain per species). Absorbance measurements were taken in 45 min intervals over 24 h. Optical density readings were plotted versus time to calculate growth kinetic parameters. C. jejuni exhibited the longest lag phase (p < 0.001) at 15 h 20 min ± 30 min, versus C. coli at 11 h 15 min ± 17 min, and C. lari at 9 h 27 min ± 15 min. The exponential phase duration was no longer than 5 h for all species, and doubling times were all less than 1h 30 min. The variation in growth kinetics for the three species of Campylobacter illustrates the importance of determining individual Campylobacter spp. growth responses for optimizing detection based on low bacterial levels. This study provides kinetics and estimates to define enrichment times necessary for low concentration Campylobacter detection.

Level of Detection (LOD50) of Campylobacter Is Strongly Dependent on Strain, Enrichment Broth, and Food Matrix

The detection of thermotolerant Campylobacter in food may be difficult due to the growth of extended-spectrum β-lactamase (ESBL)-producing Enterobacteriaceae during enrichment, resulting in false-negative samples. Therefore, the ISO protocol (ISO 10272-1:2017) suggests that, next to Bolton broth (BB), Preston broth (PB) is used as an enrichment broth to inhibit competitive flora in samples with suspected high levels of background microorganisms, such as ESBL-producing bacteria. However, the application of the strains used for validation of this ISO was not clearly characterized. This study examined the LOD₅₀ (level of detection, the concentration where the probability of detection is 50%) of the validation strains (three C. jejuni and two C. coli strains) in BB and PB using different food matrices, namely, raw milk, chicken skin, frozen minced meat, and frozen spinach. The LOD₅₀ was calculated by inoculating multiple portions with at least two inoculum levels. For each reproduction, eight test portions were used for each inoculum level and the test portion size was 10 g (chicken skin, frozen minced meat, and frozen spinach) or 10 mL (raw milk). Furthermore, the effect of artificially inoculated ESBL-producing E. coli on the LOD₅₀ was examined to mimic the presence of ESBL-producing background microorganisms in the food matrices, namely, raw milk and chicken skin. In BB, the LOD₅₀ of all strains tested in raw milk, chicken skin, and frozen spinach was rather low (0.4–37 CFU/test portion), while the LOD₅₀ in frozen minced meat was higher and much more variable (1–1,500 CFU/test portion), depending on the strain. Generally, enrichment in PB resulted in higher LOD₅₀ than in BB, especially for C. coli. Co-inoculation with ESBL-producing E. coli increased the LOD₅₀ in BB, while PB successfully inhibited the growth of this competitive microorganism. In conclusion, food matrix and enrichment broth may have a large influence on the LOD₅₀ of different Campylobacter strains. Therefore, it is not possible to give an unequivocal advice on when to use which enrichment broth, and this advocates the use of both methods in case of doubt. Furthermore, this study indicates specific strains that would be a good choice to use for Campylobacter method verification as described in ISO 16140-3:2021.

Challenging the "gold standard" of colony-forming units - Validation of a multiplex real-time PCR for quantification of viable Campylobacter spp. in meat rinses

Campylobacter jejuni is the leading bacterial food-borne pathogen in Europe. Despite the accepted limits of cultural detection of the fastidious bacterium, the "gold standard" in food microbiology is still the determination of colony-forming units (CFU). As an alternative, a live/dead differentiating qPCR has been established, using propidium monoazide (PMA) as DNA-intercalating crosslink agent for inactivating DNA from dead, membrane-compromised cells. The PMA treatment was combined with the addition of an internal sample process control (ISPC), i.e. a known number of dead C. sputorum cells to the samples. The ISPC enables i), monitoring the effective reduction of dead cell signal by the light-activated DNA-intercalating dye PMA, and ii), compensation for potential DNA losses during processing. Here, we optimized the method for routine application and performed a full validation of the method according to ISO 16140-2:2016(E) for the quantification of live thermophilic Campylobacter spp. in meat rinses against the classical enumeration method ISO 10272-2:2017. In order to render the method applicable and cost-effective for practical application, the ISPC was lyophilized to be distributable to routine laboratories. In addition, a triplex qPCR was established to simultaneously quantify thermophilic Campylobacter, the ISPC and an internal amplification control (IAC). Its performance was statistically similar to the two duplex qPCRs up to a contamination level of 4.7 log₁₀Campylobacter per ml of meat rinse. The limit of quantification (LOQ) of the alternative method was around 20 genomic equivalents per PCR reaction, i.e. 2.3 log₁₀ live Campylobacter per ml of sample. The alternative method passed a relative trueness study, confirming the robustness against different meat rinses, and displayed sufficient accuracy within the limits set in ISO 16140-2:2016(E). Finally, the method was validated in an interlaboratory ring trial, confirming that the alternative method was fit for purpose with a tendency of improved repeatability and reproducibility compared to the reference method for CFU determination. Campylobacter served as a model organism, challenging CFU as "gold standard" and could help in guidance to the general acceptance of live/dead differentiating qPCR methods for the detection of food-borne pathogens.

Combined Loop-Mediated Isothermal Amplification Assays for Rapid Detection and One-Step Differentiation of Campylobacter jejuni and Campylobacter coli in Meat Products

A loop-mediated isothermal amplification (LAMP) assay system was established, allowing rplD gene-based simultaneous detection of Campylobacter jejuni and Campylobacter coli in enriched meat products. Additionally, one-step differentiation of target species on agar plates was enabled by cdtC gene- and gyrA gene-based duplex LAMP. Both the rplD and cdtC–gyrA LAMP assays amplified the target sequences in all 62 C. jejuni and 27 C. coli strains used for determining inclusivity and revealed 100% exclusivity toward 85 tested non-target species. Throughout the entire experiments, C. jejuni and C. coli strains were 100% distinguishable by melting curves of cdtC and gyrA LAMP products. After 24-h enrichment, the rplD LAMP assay reliably detected initial inoculation levels of 10–100 CFU/g in artificially contaminated minced meat. Investigation of naturally contaminated meat samples revealed a diagnostic accuracy of 95% toward real-time PCR and 94.1% toward the standard culture method applying the 24-h incubation period. Diagnostic sensitivity and specificity, and positive and negative predictive values were 89.8, 100, 100, and 91.2%, respectively, when measured against real-time PCR, and 89.6, 98.1, 97.7, and 91.2%, respectively, when measured against the standard culture method. After 48-h enrichment, the detection limit of the rplD LAMP assay improved to initial inoculation levels of 1–10 CFU/g in artificially contaminated minced meat. Applying the 48-h incubation period on naturally contaminated meat samples resulted in 100% concordant results between rplD LAMP, real-time PCR, and the standard culture method. The established LAMP assay system was proved to be suitable for rapid meat sample screening. Furthermore, it constitutes a promising tool for investigating other Campylobacter sources and could therefore make a valuable contribution to protect consumers from foodborne illness.