What are Viable but Nonculturable (VBNC) cells and why they pose a threat to public health

 

Contributed by Narine Arabyan 

Viable but nonculturable (VBNC) state was first discovered in 1982 in the lab of Rita Colwell at University of Maryland. VBNC state in bacteria is induced by many different environmental stresses (nutrient starvation, temperature shifts, oxidative and osmotic stress and etc.). Before the discovery of VBNC state, bacterial cells were considered dead when it was unable to grow on culture media. However, we know now that cells can stop growing on media but remains viable and potentially able to regrow. VBNC state in bacteria is characterized as living cells that are unable to grow on routine media on which they normally grow, have low levels of metabolic activity and become culturable when resuscitated.

VBNC cells have many differences from the viable culturable cells. The differences are found in their cellular morphology, cell wall, metabolism, gene expression, physical and chemical resistance, adhesion properties and virulence potential. VBNC cells undergo cell size reduction in cell size and cells become round. The fatty acids and peptidoglycan change in the cell wall. There is an increase in the unsaturated fatty acids and carbon chains become less than 16 carbons. There is an increase in the peptidoglycan cross-linking which is thought to strengthen the cell wall. VBNC cells have greater physical and chemical resistance which could be due to cell wall strengthening. VBNC are able to tolerate heat, low pH, ethanol, antibiotics and heavy metals. VBNC cells are capable of retaining attachment ability and are still virulent due to faster resuscitation into culturable cells upon introduction to proper host.

Cells enter VBNC state under stressful conditions. This is an adaptive strategy for long-term survival and is advantages for bacteria. Bacteria in VBNC state stay alive for long time until conditions become favorable. The ability to enter the VBNC state poses a risk to public health. This is a serious problem for pathogen detection. The inability to culture causes problems for proper diagnosis of diseases and subsequent treatments. This is a huge problem for quality control in the food industry and water distribution systems. Risks become apparent even more when pathogenic bacteria in the VBNC state regain virulence after resuscitation. Although this state in bacteria has been discovered more than 30 years ago, we still don’t know a lot about VBNC state in bacteria.

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