Cultured microbes tend to lose their toxin production over time.
Article Propagation of Salmonella 3, 10: r in ileum: a transmission ...
It is a selection for the fittest pathogen. When you have selective pressure on the pathogen, the most virulent pathogen survives. Whereas in culture, there is no selective pressure and hence the pathogen loses its virulence (eg: toxin production)
Dear Rakesh, Bacteria that are exposed to a noxious agent will very quickly respond with the over-expression of efflux pumps that extrude the noxious agent before it reaches a sensitive target. When the concentration of that agent is increased incrementally, more efflux pumps are made. However, when the organism is then maintained in the last concentration of the agent for a prolonged period of time, a mutator master gene is activated and a series of mutations of essential genes takes place (30 S ribosome; membrane sensitive targets such as those affected by beta-lactams; gyrase; etc). In other words, when the bacterium is faced with an unchanging environment that contains a constant concentration of a given noxious agent, it changes from an efflux type of response which is very energy consuming to a permanent mutational response that results in the survival of the bacterium. However, if confronted with competition with a wild type strain, it will not survive. I have attached a number of our studies that may assist you with the concept.
Here are the references:
Martins A, Hunyadi A, Amaral L. Mechanisms of resistance in bacteria: an evolutionary approach. Open Microbiol J. 2013;7:53-8.
http://www.ncbi.nlm.nih.gov/pubmed/23560029
Martins A, Spengler G, Molnar J and Amaral L. Sequential responses of bacteria to noxious agents (antibiotics) leading to accumulation of mutations and permanent resistance. Biochemistry & Pharmacology Journal Open Access. 2012; 1:7.
http://dx.doi.org/10.4172/2167-0501.1000104
Martins A, Amaral L. Screening for efflux pump systems of bacteria by the new acridine orange agar method. In Vivo. 2012;26:203-206.
http://www.ncbi.nlm.nih.gov/pubmed/22351659
Martins A Spengler G, Rodrigues L, Viveiros M, Ramos J, Martins M, Couto I, Fanning S, Pagès JM, Bolla JM, Molnar J and Amaral L. pH Modulation of Efflux Pump Activity of Multi-Drug Resistant E. coli: Protection During its Passage and Eventual Colonization of the Colon. PLoS One 2009;4:e6656.
http://www.ncbi.nlm.nih.gov/pubmed/19684858
Martins A, Spengler G, Rodrigues L, Viveiros M, Ramos J, Martins M, Couto I, Fanning S, Pages JM, Bolla JM and Amaral L. AcrAB mediated MDR phenotype is maintained after efflux pump genes and their regulators have restored wild type activities: International Journal Antimicrobial Agents 2009;34:602-604.
http://www.ncbi.nlm.nih.gov/pubmed/19734019
Leonard, thanks for comprehensive answer and your nice work references.
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