Pyruvate relieves the necessity of high induction levels of catalase and enables Campylobacter jejuni to grow under fully aerobic conditions

Aerotolerancy of Campylobacter spp.: A Comprehensive Review

Campylobacter spp. constitute a group of microaerophilic bacteria that includes strains that are aerotolerant and capable of surviving in aerobic conditions. Recent studies have shown that aerotolerant strains are highly prevalent in meats, animals, and clinical settings. Changes in growth media and other environmental conditions can affect the aerotolerance of Campylobacter strains and must be considered when studying their aerotolerance in vitro. Polymicrobial interactions and biofilms also play a significant role in the ability of Campylobacter to survive oxygen exposure. Continuous subculturing may foster aerotolerance, and studies have demonstrated a positive correlation between aerotolerance and virulence and between aerotolerance and the ability to survive stressful environmental conditions. Various mechanisms and genetic origins for aerotolerance have been proposed; however, most of the potential genes involved in aerotolerance require further investigation, and many candidate genes remain unidentified. Research is also needed to investigate if there are any clinical implications for Campylobacter aerotolerance. Understanding the aerotolerance of Campylobacter remains an important target for further research, and it will be an important step towards identifying potential targets for intervention against this clinically important food-borne pathogen.

Characterization of viable but nonculturable state of Campylobacter concisus

Campylobacter concisus is an opportunistic bacterial pathogen linked with a range of human diseases. The objective of this study was to investigate the viable but nonculturable (VBNC) state of the bacterium. To induce the VBNC state, C. concisus cells were maintained in sterilized phosphate-buffered saline at 4°C for three weeks. The VBNC cells were monitored using quantitative analysis by propidium monoazide (PMAxx) coupled with quantitative real-time PCR (PMAxx-qPCR), targeting the DNA gyrase subunit B gene. The results demonstrated that C. concisus ATCC 51562 entered the VBNC state in 15 days, while ATCC 51561 entered the VBNC state in 9 days. The viable cell counts, assessed by PMAxx-qPCR, consistently remained close to the initial level of 107 CFU ml-1, indicating a substantial portion of the cell population had entered the VBNC state. Notably, morphological analysis revealed that the VBNC cells became coccoid and significantly smaller. The cells could be resuscitated through a temperature increase in the presence of a highly nutritious growth medium. In conclusion, under environmental stress, most C. concisus cells converted to the VBNC state. The VBNC state of C. concisus may be important for its environmental survival and spread, and the presence of VBNC forms should be considered in environmental and clinical monitoring.

Enhancement of Campylobacter hepaticus culturing to facilitate downstream applications

Campylobacter hepaticus causes Spotty Liver Disease (SLD) in chickens. C. hepaticus is fastidious and slow-growing, presenting difficulties when growing this bacterium for the preparation of bacterin vaccines and experimental disease challenge trials. This study applied genomic analysis and in vitro experiments to develop an enhanced C. hepaticus liquid culture method. In silico analysis of the anabolic pathways encoded by C. hepaticus revealed that the bacterium is unable to biosynthesise L-cysteine, L-lysine and L-arginine. It was found that L-cysteine added to Brucella broth, significantly enhanced the growth of C. hepaticus, but L-lysine or L-arginine addition did not enhance growth. Brucella broth supplemented with L-cysteine (0.4 mM), L-glutamine (4 mM), and sodium pyruvate (10 mM) gave high-density growth of C. hepaticus and resulted in an almost tenfold increase in culture density compared to the growth in Brucella broth alone (log10 = 9.3 vs 8.4 CFU/mL). The type of culture flask used also significantly affected C. hepaticus culture density. An SLD challenge trial demonstrated that C. hepaticus grown in the enhanced culture conditions retained full virulence. The enhanced liquid culture method developed in this study enables the efficient production of bacterial biomass and therefore facilitates further studies of SLD biology and vaccine development.

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

Campylobacter jejuni and Campylobacter coli continue to be the leading cause of zoonotic gastroenteritis in the European Union, making reliable detection in food important. Low storage temperatures and atmospheric oxygen concentrations during food production can cause sub-lethal damage or transient non-culturability which is why ISO 10272-1:2017 includes an enrichment step to repair cell damage and increase cell concentrations, thereby supporting detection of campylobacters from foods. The aim of this study was to assess the variability in lag-duration of C. jejuni and C. coli during enrichment after different food-relevant stress treatments and evaluate its impact on growth kinetics and reliability of detection outcomes. Therefore, 13 C. jejuni and 10 C. coli strains were subjected to cold stress during refrigerated and frozen storage. Refrigerated storage did not significantly reduce culturability, but frozen storage reduced cell concentrations by 1.6 ± 0.1 log₁₀cfu/ml for both species. Subsequently, cells were enriched following ISO 10272-1:2017-A and cell concentrations were determined over time and lag-duration and growth rate were determined by fitting the Baranyi-model. Without prior stress treatment, mean lag-duration for C. jejuni and C. coli was 2.5 ± 0.2 h and 2.2 ± 0.3 h, respectively. Refrigerated storage increased lag-duration for C. jejuni to 4.6 ± 0.4 h and for C. coli to 5.0 ± 0.4 h and frozen storage increased lag-duration to 5.0 ± 0.3 h and 6.1 ± 0.4 h for C. jejuni and C. coli, respectively. Comparison of strain- and biological variability showed that differences in recovery after cold stress can be attributed mainly to strain variability since strain variability after refrigeration and freeze stress increased respectively 3-fold and 4-fold while biological variability remained constant. A subset of strains was also subjected to oxidative stress that reduced cell concentrations by 0.7 ± 0.2 log₁₀ cfu/ml and comparison of recovery patterns after oxidative and freeze stress indicated that recovery behaviour was also dependent on the stress applied. A scenario analysis was conducted to evaluate the impact of heterogeneity in outgrowth kinetics of single cells on the reliability of detection outcomes following ISO protocol 10272-1:2017. This revealed that a 'worst-case'-scenario for successful detection by a combination of the longest lag-duration of 7.6 h and lowest growth rate of 0.47 h^-1 still resulted in positive detection outcomes since the detection limit was reached within 32.5 h. This suggests that other factors such as competitive microbiota can act as a causative factor in false-negative outcomes of tested food samples.

A Novel Highly Efficient Device for Growing Micro-Aerophilic Microorganisms

This work describes a novel, simple and cost-effective culture system, named the Micro-Oxygenated Culture Device (MOCD), designed to grow microorganisms under particularly challenging oxygenation conditions. Two microaerophilic magnetotactic bacteria, a freshwater Magnetospirillum gryphiswaldense strain MSR-1 and a marine Magnetospira sp. strain QH-2, were used as biological models to prove the efficiency of the MOCD and to evaluate its specifications. Using the MOCD, growth rates of MSR-1 and QH-2 increased by four and twofold, respectively, when compared to traditional growing techniques using simple bottles. Oxystat-bioreactors have been typically used and specifically designed to control low dissolved oxygen concentrations, however, the MOCD, which is far less sophisticated was proven to be as efficient for both MSR-1 and QH-2 cultures with regard to growth rate, and even better for MSR-1 when looking at cell yield (70% increase). The MOCD enables a wide range of oxygenation conditions to be studied, including different O₂-gradients. This makes it an innovative and ingenious culture device that opens up new parameters for growing microaerobic microorganisms.

Bacillus megaterium adapts to acid stress condition through a network of genes: Insight from a genome-wide transcriptome analysis

RNA-seq analysis of B. megaterium exposed to pH 7.0 and pH 4.5 showed differential expression of 207 genes related to several processes. Among the 207 genes, 11 genes displayed increased transcription exclusively in pH 4.5. Exposure to pH 4.5 induced the expression of genes related to maintenance of cell integrity, pH homeostasis, alternative energy generation and modification of metabolic processes. Metabolic processes like pentose phosphate pathway, fatty acid biosynthesis, cysteine and methionine metabolism and synthesis of arginine and proline were remodeled during acid stress. Genes associated with oxidative stress and osmotic stress were up-regulated at pH 4.5 indicating a link between acid stress and other stresses. Acid stress also induced expression of genes that encoded general stress-responsive proteins as well as several hypothetical proteins. Our study indicates that a network of genes aid B. megaterium G18 to adapt and survive in acid stress condition.

Epithelial Coculture and l-Lactate Promote Growth of Helicobacter cinaedi under H2-Free Aerobic Conditions

Helicobacter cinaedi is an emerging opportunistic pathogen associated with infections of diverse anatomic sites. Nevertheless, the species demonstrates fastidious axenic growth; it has been described as requiring a microaerobic atmosphere, along with a strong preference for supplemental H₂ gas. In this context, we examined the hypothesis that in vitro growth of H. cinaedi could be enhanced by coculture with human epithelial cells. When inoculated (in Ham's F12 medium) over Caco-2 monolayers, the type strain (ATCC BAA-847) gained the ability to proliferate under H₂-free aerobic conditions. Identical results were observed during coculture with several other monolayer types (LS-174T, AGS, and HeLa). Under chemically defined conditions, 40 amino acids and carboxylates were screened for their effect on the organism's atmospheric requirements. Several molecules promoted H₂-free aerobic proliferation, although it occurred most prominently with millimolar concentrations of l-lactate. The growth response of H. cinaedi to Caco-2 cells and l-lactate was confirmed with a collection of 12 human-derived clinical strains. mRNA sequencing was next performed on the type strain under various growth conditions. In addition to providing a whole-transcriptome profile of H. cinaedi, this analysis demonstrated strong constitutive expression of the l-lactate utilization locus, as well as differential transcription of terminal respiratory proteins as a function of Caco-2 coculture and l-lactate supplementation. Overall, these findings challenge traditional views of H. cinaedi as an obligate microaerophile.

IMPORTANCE

H. cinaedi is an increasingly recognized pathogen in people with compromised immune systems. Atypical among other members of its bacterial class, H. cinaedi has been associated with infections of diverse anatomic sites. Growing H. cineadi in the laboratory is quite difficult, due in large part to the need for a specialized atmosphere. The suboptimal growth of H. cinaedi is an obstacle to clinical diagnosis, and it also limits investigation into the organism's biology. The current work shows that H. cinaedi has more flexible atmospheric requirements in the presence of host cells and a common host-derived molecule. This nutritional interplay raises new questions about how the organism behaves during human infections and provides insights for how to optimize its laboratory cultivation.

Influence of measurement and control of microaerobic gaseous atmospheres in methods for Campylobacter growth studies

Campylobacter is the leading cause of bacterial enteritis in the world. For this reason, this pathogen is widely studied. As a microaerophilic and capnophilic microorganism, this foodborne pathogen requires an atmosphere with reduced oxygen (O2) and elevated carbon dioxide (CO2) concentrations for its optimal growth in vitro. According to the procedure for Campylobacter spp. isolation and cultivation from food products and environmental samples, European and American standards recommend gas proportions of 5% O2 and 10% CO2, complemented with nitrogen (N2). However, in the literature, the reported proportion of O2 for microaerobic growth conditions of Campylobacter spp. can range from 2.5% to 15% and the reason for this variation is usually not explained. The use of different gas generating systems and media to detect and to grow Campylobacter from foodstuff and the lack of information about gas producing systems are the main sources of the loss of consistancy between data. In this review, the relevance, strengths and weaknesses of these methods and their impact on Campylobacter biology are discussed. In conclusion the minimum information concerning microaerobic gaseous atmospheres are suggested in order to better harmonize data obtained from research studies for a better understanding of Campylobacter features.

Refined analysis of the Campylobacter jejuni iron-dependent/independent Fur- and PerR-transcriptomes

Background

The genome of Campylobacter jejuni contains two iron activated Fur-family transcriptional regulators, CjFur and CjPerR, which are primarily responsible for regulating iron homeostasis and oxidative stress respectively. Both transcriptional regulators have been previously implicated in regulating diverse functions beyond their primary roles in C. jejuni. To further characterize their regulatory networks, RNA-seq was used to define the transcriptional profiles of C. jejuni NCTC11168 wild type, Δfur, ΔperR and ΔfurΔperR isogenic deletion mutants under both iron-replete and iron-limited conditions.

Results

It was found that 202 genes were differentially expressed in at least one mutant under iron-replete conditions and 331 genes were differentially expressed in at least one mutant under iron-limited conditions. The CjFur and CjPerR transcriptomes characterized in this study were compared to those previously identified using microarray profiling and found to be more extensive than previously understood. Interestingly, our results indicate that CjFur/CjPerR appear to co-regulate the expression of flagellar biogenesis genes in an opposing and iron-independent fashion. Moreover the ΔfurΔperR isogenic deletion mutant revealed that CjFur and CjPerR can compensate for each other in certain cases, suggesting that both regulators may compete for binding to specific promoters.

Conclusions

The CjFur and CjPerR transcriptomes are larger than previously reported. In particular, deletion of perR results in the differential expression of a large group of genes in the absence of iron, suggesting that CjPerR may also regulate genes in an iron-independent manner, similar to what has already been demonstrated with CjFur. Moreover, subsets of genes were found which are only differentially expressed when both CjFur and CjPerR are deleted and includes genes that appear to be simultaneously activated by CjFur and repressed by CjPerR. In particular the iron-independent co-regulation of flagellar biogenesis by CjFur/CjPerR represents a potentially novel regulatory function for these proteins. These findings represent additional modes of co-regulation by these two transcriptional regulators in C. jejuni.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1661-7) contains supplementary material, which is available to authorized users.

Impact of oxidative stress defense on bacterial survival and morphological change in Campylobacter jejuni under aerobic conditions

Campylobacter jejuni, a microaerophilic foodborne pathogen, inescapably faces high oxygen tension during its transmission to humans. Thus, the ability of C. jejuni to survive under oxygen-rich conditions may significantly impact C. jejuni viability in food and food safety as well. In this study, we investigated the impact of oxidative stress resistance on the survival of C. jejuni under aerobic conditions by examining three mutants defective in key antioxidant genes, including ahpC, katA, and sodB. All the three mutants exhibited growth reduction under aerobic conditions compared to the wild-type (WT), and the ahpC mutant showed the most significant growth defect. The CFU reduction in the mutants was recovered to the WT level by complementation. Higher levels of reactive oxygen species were accumulated in C. jejuni under aerobic conditions than microaerobic conditions, and supplementation of culture media with an antioxidant recovered the growth of C. jejuni. The levels of lipid peroxidation and protein oxidation were significantly increased in the mutants compared to WT. Additionally, the mutants exhibited different morphological changes under aerobic conditions. The ahpC and katA mutants developed coccoid morphology by aeration, whereas the sodB mutant established elongated cellular morphology. Compared to microaerobic conditions, interestingly, aerobic culture conditions substantially induced the formation of coccoidal cells, and antioxidant treatment reduced the emergence of coccoid forms under aerobic conditions. The ATP concentrations and PMA–qPCR analysis supported that oxidative stress is a factor that induces the development of a viable-but-non-culturable state in C. jejuni. The findings in this study clearly demonstrated that oxidative stress resistance plays an important role in the survival and morphological changes of C. jejuni under aerobic conditions.

The transcriptional landscape of Campylobacter jejuni under iron replete and iron limited growth conditions

The genome-wide Campylobacter jejuni transcriptional response under iron replete and iron limited conditions was characterized using RNA-seq. We have identified 111 novel C. jejuni 5’UTRs and mapped 377 co-transcribed genes into 230 transcriptional operons. In contrast to previous microarray results, the C. jejuni iron stimulon is less extensive than previously believed and consists of 77 iron activated genes and 50 iron repressed genes. As anticipated, the iron repressed genes are primarily those involved in iron acquisition or oxidative stress defense. Interestingly, these experiments have revealed that iron is an important modulator of flagellar biogenesis with almost all the components of the flagella found to be iron activated. Given that motility is a well-known C. jejuni colonization factor, this suggests that there is an important regulatory coupling of flagellar biogenesis and iron level in C. jejuni. In addition we have identified several consensus mutations in the C. jejuni NCTC11168 strain that are widespread in the Campylobacter research community and which may explain conflicting phenotypic reports for this strain. Comparative analysis of iron responsive genes with the known Fur regulon indicates that many iron responsive genes are not Fur responsive; suggesting that additional iron regulatory factors remain to be characterized in C. jejuni. Further analysis of the RNA-seq data identified multiple novel transcripts including 19 potential ncRNAs. The expression of selected ncRNAs was confirmed and quantified with qRT-PCR. The qRT-PCR results indicate that several of these novel transcripts are either Fur and/or iron responsive. The fact that several of these ncRNAs are iron responsive or Fur regulated suggests that they may perform regulatory roles in iron homeostasis.

Enrichment culture can bias the isolation ofCampylobactersubtypes

Enrichment culture is often used to isolateCampylobacter. This study compared isolation ofCampylobacterspp. from 119 broiler chicken environments from two farms, using Preston and modified Exeter (mExeter) and modified Bolton (mBolton) enrichments. mExeter was significantly more effective in isolatingCampylobacterspp. from the environmental samples compared to Preston (P<0·001) and mBolton (P<0·04) broths but there was no significant difference between the latter two methods (P>0·05). Enrichment broth type did not affect isolation from chicken faecal or soil and litter samples.C. jejuniwas isolated from significantly more environmental samples using mExeter broth compared to Preston (P<0·01) and mBolton (P<0·003) broths; there was no difference between the latter two methods or between all methods for detection ofC. coli(P>0·05). OnlyC. coliwas isolated from the soil and litter samples and although bothC. jejuniandC. coliwere recovered from the faecal samples there was no effect of using different enrichment broths. The majority of samples where the same species had been isolated yielded the same or closely related genotypes as defined by pulsed-field gel electrophoresis. Isolates recovered using Preston and mBolton broths were less genetically diverse than those from mExeter broth. We conclude that the enrichment method used affects both the number and species ofCampylobacterisolated from naturally contaminated samples.

Different contributions of HtrA protease and chaperone activities to Campylobacter jejuni stress tolerance and physiology

The microaerophilic bacterium Campylobacter jejuni is the most common cause of bacterial food-borne infections in the developed world. Tolerance to environmental stress relies on proteases and chaperones in the cell envelope, such as HtrA and SurA. HtrA displays both chaperone and protease activities, but little is known about how each of these activities contributes to stress tolerance in bacteria. In vitro experiments showed temperature-dependent protease and chaperone activities of C. jejuni HtrA. A C. jejuni mutant lacking only the protease activity of HtrA was used to show that the HtrA chaperone activity is sufficient for growth at high temperature or under oxidative stress, whereas the HtrA protease activity is essential only under conditions close to the growth limit for C. jejuni. However, the protease activity was required to prevent induction of the cytoplasmic heat shock response even under optimal growth conditions. Interestingly, the requirement of HtrA at high temperatures was found to depend on the oxygen level, and our data suggest that HtrA may protect oxidatively damaged proteins. Finally, protease activity stimulates HtrA production and oligomer formation, suggesting that a regulatory role depends on the protease activity of HtrA. Studying a microaerophilic organism encoding only two known periplasmic chaperones (HtrA and SurA) revealed an efficient HtrA chaperone activity and proposed multiple roles of the protease activity, increasing our understanding of HtrA in bacterial physiology.

High hydrostatic pressure resistance of Campylobacter jejuni after different sublethal stresses

AIMS

To study the development of resistance responses in Campylobacter jejuni to high hydrostatic pressure (HHP) treatments after the exposure to different stressful conditions that may be encountered in food-processing environments, such as acid pH, elevated temperatures and cold storage.

METHODS AND RESULTS

Campylobacter jejuni cells in exponential and stationary growth phase were exposed to different sublethal stresses (acid, heat and cold shocks) prior to evaluate the development of resistance responses to HHP. For exponential-phase cells, neither of the conditions tested increased nor decreased HHP resistance of C. jejuni. For stationary-phase cells, acid and heat adaptation-sensitized C. jejuni cells to the subsequent pressure treatment. On the contrary, cold-adapted stationary-phase cells developed resistance to HHP.

CONCLUSIONS

Whereas C. jejuni can be classified as a stress sensitive micro-organism, our findings have demonstrated that it can develop resistance responses under different stressing conditions. The resistance of stationary phase C. jejuni to HHP was increased after cells were exposed to cold temperatures.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results of this study contribute to a better knowledge of the physiology of C. jejuni and its survival to food preservation agents. Results here presented may help in the design of combined processes for food preservation based on HHP technology.

Oxidative stress in Campylobacter jejuni: responses, resistance and regulation

Campylobacter jejuni is a major food-borne human pathogen that paradoxically is an oxygen-sensitive microaerophile, yet must resist the oxidative stresses encountered both in the host and in the environment. Recent studies suggest that, perhaps surprisingly, C. jejuni contains a wide range of enzymes involved in oxidative stress defense, and this review focuses on the properties and roles of these proteins. Although the mechanisms of gene regulation are still poorly understood in C. jejuni, several regulators of the oxidative stress response have been identified and their properties are discussed here. We suggest that future studies should be directed towards identifying the role of additional and less well characterized components involved in oxidative stress resistance, as well as providing a more complete picture of the underlying sensing and regulatory mechanisms.