Molecular genetics of salmonela survival and resistance
Authors: Rublenko N. 
Published: July 2019

Salmonella is one of the most common cause of the food borne illness. Salmonella belongs to Enterobacteriacae family and consists of 2 species, which diverge on 6 subspecies.These subspecies consists of 2700 serovars. There are typhoid serovars among them - S. Typhi, Paratyphi A, B, C - which cause typhoid fever in human. The rest of the serovars are non-typhoidal and leads to gastroenteritis both in animal and human. Salmonella enters to a mammal organism as a result of consumption of contaminated food products: meat, eggs, milk and products containing them. The entry of the infection for salmonellosis is the small intestine mucosa. Salmonella attaches to cell walls by fimbria and pili.
Salmonella has several systems that are activated in response to adverse conditions such as: high osmolarity, acid or heat shock and nutrient deficiencies. They are based on the principle of a two-component system in which there is a sensor that receives cytoplasmic signals, and a regulator. Regulator (usually DNA-binding protein) initiates the transcription of the virulence genes (Chakraborty, 2015). The sensor is histidine kinase, which phosphorylates the regulatory protein, thereby activating it.During the infectious cycle of salmonella in mammalian organism the formation of specific vacuole SCV takes place (Salmonella-containing vacuole-containing vacuole containing salmonella) in the cytoplasm of the eukaryotic cell (Steele-Mortimer, 2008). SCV is a modified phagosome, which is formed as a result of cytoskeleton rearrangements. The target are usually phagocytic cells : neutrophils, macrophages and epithelial cells of the small intestine mucosa - M-cells (Akhmetova, 2012).
Given the specific mechanism of infection, salmonella is considered a facultative intracellular pathogen. Bacterium invades the eukaryotic cell by rearrangement of its cytoskeleton with effector proteins and continue to persistence in a form of SCV.
It is well-known nowadays that tolerance to high osmotic pressure is achieved through the EnvZ / OmpR system, which also regulates the expression of the ssrAB operon that is localized on the Salmonella pathogenicity island SPI-2 and triggers the expression of the effector proteins. The ssrAB operon is also regulated by the two-component acid shock response system PhoP / PhoQ (Worley, 2000). The functioning of the PhoP / PhoQ system directly depends on the sigma factor RpoS, which accumulates under low concentrations of magnesium cations (Tu, 2006).
According to the researches of transduction between the EnvZ / OmpR components, it is clear that salmonella receives signals from the cytoplasmic environment, and sensory molecules are located on the inner membrane (Kenney, 2019; Wang et al., 2012).
The ability to survive under acid shock is provided by the PhoP / PhoQ system, which also operates on the principle of signal transduction. PhoQ is a Histidine Kinase Signal Sensor. Signals are acidic pH, divalent cations and positively charged antimicrobial peptides.
An important function of the two-component PhoP / PhoQ system is the control of spi ssa gene expression in a macrophage environment (Bijlsma, 2005). These genes are the main component of the type III secretion systems and are transcribed only when salmonella enters eukaryotic cell. (Bijlsma, 2005). The main regulator of signal transduction systems PhoP/PhoQ and EnvZ/OmpR is sigma-factor RpoS - subunit of bacterial RNA-polymerase - which operates in stationary phase at low pH, high omolarity, heat shock or nutrient deficiency. RpoS protein accumulates in adverse conditions during stationary phase (Mg2+ deficiency, low pH, high osmolarity). Need in magnesium cations is dependent on their ability to act as cofactors in many enzymatic activities. The accumulation begins at exponential (logarithmic) phase of bacterial reproduction. This is the phase of active cell division. Two factors MgtA and MgtB are responsible for Mg2+ transport. Another molecule with the same function is CorA - bivalent cation channel, though its transcriptions doesn’t depent on magnesium concentration in cell. In a case of magnesium deficiency at the stationary phase, RpoS accumulates vigorously an initiates replication of PhoP/PhoQ.
PhoP/PhoQ regulates tolerance to inorganic acids. Also, PhoP/PhoQ controls adaptation to magnesium cations deficiency and macrophage activity. Results of many studies on genes coding this system and their mutations led to conclusion the mutation or inactivation of one factor causes decrease in virulence and makes bacterial susceptible to acid environment.
To date, the stages of the infectious process for salmonellosis have been studied and described in detail in the literature. Particular attention is paid to signal transduction systems that are common among enterobacteria and help to avoid adverse conditions. Their functioning and regulation are investigated.
It is known that salmonella receives signals for the activation of sensors from the cytoplasm, but the nature of these signals is not yet fully understood. Adaptation of the bacteria to adverse conditions and the response to phagocytosis is initiated by the transcription of pathogenic genes and the suppression of the transcription of the operon, which neutralize the conditions in the cytoplasm of salmonella cells. Thus, adapting to the conditions of target cells, salmonella continues to multiply in the body.
Key words: salmonella, pH, osmolarity, virulencegenes, operon, signal transduction.