Cronobacter sakazakii MEMASUKI KONDISI VIABLE BUT NONCULTURABLE SELAMA PEMBENTUKAN BIOFILM

VBNC Cronobacter sakazakii survives in macrophages by resisting oxidative stress and evading recognition by macrophages

Survival in host macrophages is an effective strategy for pathogenic bacterial transmission and pathogenesis. Our previous study found that viable but non-culturable (VBNC) Cronobacter Sakazakii (C. sakazakii) can survive in macrophages, but its survival mechanism is not clear. In this study, we investigated the possible mechanisms of VBNC C. sakazakii survival in macrophages in terms of environmental tolerance within macrophages and evasion of macrophages recognition. The results revealed that VBNC C. sakazakii survived under oxidative conditions at a higher rate than the culturable C. sakazakii. Moreover, the stringent response gene (relA and spoT) and the antioxidant-related genes (sodA, katG, and trxA) were up-regulated, indicating that VBNC C. sakazakii may regulate antioxidation through stringent response. On the other hand, compared with culturable C. sakazakii, VBNC C. sakazakii caused reduced response (Toll-like receptor 4) in macrophages, which was attributed to the suppression of biosynthesis of the lipopolysaccharides (LPS). Furthermore, we found that ellagic acid can reduce the survival rate of bacteria in macrophages by improving the immune TLR4 recognition ability of macrophages. In conclusion, VBNC C. sakazakii may survive in macrophages by regulating oxidative tolerance through stringent response and altering LPS synthesis to evade TLR4 recognition by macrophages, which suggests the pathogenic risk of VBNC C. sakazakii.

Quantitative Detection of Viable but Nonculturable Cronobacter sakazakii Using Photosensitive Nucleic Acid Dye PMA Combined with Isothermal Amplification LAMP in Raw Milk

An accurate method that rapidly detects the number of viable but nonculturable (VBNC) Cronobacter sakazakii was developed by combining propidium bromide with quantitative LAMP (PMA-QLAMP). The gyrB gene was the target for primers design. The optimal PMA treatment conditions were determined to eliminate the DNA amplification of 10⁸ CFU/mL of dead C. sakazakii without affecting any viable C. sakazakii DNA amplification. Compared with the DNA of 24 strains of common non-C. sakazakii strains found in raw milk and dairy products, the DNA of only six C. sakazakii strains from different sources was amplified using PMA-QLAMP. The ability of PMA-QLAMP to quantitatively detect non-dead C. sakazakii in a 10% powdered infant formula (PIF) solution was limited to 4.3 × 10² CFU/mL and above concentrations. Pasteurizing 10⁶ CFU/mL viable C. sakazakii yielded the maximum ratio of the VBNC C. sakazakii. PMA-QLAMP-based detection indicated that, although approximately 13% of 60 samples were positive for viable C. sakazakii, the C. sakazakii titers in these positive samples were low, and none entered the VBNC state under pasteurization. PMA-QLAMP showed potential as a specific and reliable method for detecting VBNC-C. sakazakii in pasteurized raw milk, thereby providing an early warning system that indicates potential contamination of PIF.

Expression of rpoS, ompA and hfq genes of Cronobacter sakazakii strain Yrt2a during stress and viable but nonculturable state

Cronobacter spp. in powdered infant formula has been etiologically linked to meningitis and necrotizing enterocolitis in certain groups of infants. This study aimed to determine whether C. sakazakii Yrt2a strain experiencing desiccation stress could enter viable but nonculturable (VBNC) state as well as to examine the expression of genes associated with stress and virulence during the above states. Stress and VBNC conditions were determined based on viability and culturability assays. Expression of genes related to stress (rpoS) and virulence (hfq and ompA) was evaluated by real-time PCR. The results showed that C. sakazakii Yrt2a entered VBNC 24 days post exposure to 2 h of desiccation treatment. The expression of rpoS, hfq and ompA genes was up-regulated during stress conditions, suggesting that Cronobacter successfully managed stress to maintain its culturability while maintaining its virulence. The expression of the target genes decreased at VBNC state but remained higher than that of a normal state. These findings reinforce the assumption that C. sakazakii undergoing VBNC state maintains its pathogenicity.