This paper indicates that not all types of Gram-negative bacteria are capable of adaptation or resistance learning against gamma radiation at least at the mentioned level of doses and with this method of experiment (it might be different for more cycles and/or variety doses of irradiation). As mentioned in the results, only three types of the studied group adapted themselves to resist more than the first exposure against the same amount of radiation. This experiment is suggesting that for any purpose to be accomplished using Gram-negative bacteria in a radiation exposed environment, these types of bacteria can become resistant to the proper dose of radiation in a relatively short period of time. NASA, for instance, has invested in a variety of research to evaluate the harmfulness of certain bacteria in space stations and to conduct a conclusion on their adaptability in outer space. In order to fully explore the details regarding the effects of gamma radiation on the bacteria in this experiment, different components of radiation resistance are to be surveyed separately. First, we will discuss the cell wall and outer membrane. It is mentioned in a research that Gram-positive bacteria are more resistant to UV radiation than Gram-negative ones (Romanovskaya et al. 2011). It is also discussed in a similar experiment (Williams et al. 2007) that for the germicidal radiation (UV-C) Gram-positive bacteria showed 12-13 times more resistance. This can be explained by the fact that the Gram-positive class possesses a thick cell wall (higher amount of peptidoglycan) that changes the cell susceptibilities to various environmental conditions (Mai-Prochnow et al. 2016). Similar behavior is observed in experiments regarding gamma radiation resistance (Anellis et al. 1973). This is also most importantly caused by the sulfur compounds found in the cell wall (Braun et al. 1996; Milligan et al. 1997). It is important to notice that Gram-negative bacteria, although containing a thin cell wall, still do possess this capability. Peptidoglycan recycling is also a metabolic process by which Gram-negative bacteria are able to show resistance (Mayer et al. 2019). Microscopic physical reasons for this phenomenon need further investigations concerning different physical properties of the UV light and gamma radiation interactions with bacteria. Lipopolysaccharide is another important component of the Gram-negative bacteria that is to be considered. It is known that phosphate groups of the lipopolysaccharides increase the overall negative charge, this negative charge, similar to sulfur compounds, help to stabilize the whole structure (Herrera et al. 2010) and can be consequently resistant to free radicals produced by gamma radiation. The fact that Salmonella is much more resistant to many antibiotics that affect cell wall synthesis (V T Nair et al. 2018; Lee 2011) (relative to other Gram-negative bacteria) indicates that its peptidoglycan recycling process and outer membrane’s negative charge is relatively strong, hence, this is probably one of the reasons that S.Typhi is also more resistant to gamma radiation.
After covering the factors related to cell walls that were involved in radiation resistance in our results, here we discuss the genetic and DNA repair factors. It is known that oxidative DNA damage is a result of ionizing radiation. A frequent outcome of this damage is the production of 7,8-dihydro-8-oxoguanine (GO), a mutagenic base analog (Shibutani et al. 1991), and protection against this base relies on the GO system, composed of three genes: mutM, mutT, and mutY which are present in S.Typhi as fpg. Oxidative damages induce a variety of defense mechanisms that in case of some mutations of Salmonella Typhimurium are sometimes lacking certain pathways (Garzón et al. 1996; Kokubo et al. 2005), which might be one of the most important reasons that no adapting capability was observed for this strain in this study; however, cases of radiation resistant strains of Salmonella Typhimurium were reported and important courses regarding their resistance were discussed (Licciardello et al 1969; Davies and Sinskey 1973). That can be explained by different environmental conditions and / or divergent genomes. There are explanations asserting the reason for E.Coli and Serratia strains being most susceptible to gamma irradiation. Since the relations between beta lactamase enzymes and radiation resistance are uncovered recently (Gaougaou et al. 2018; Yehia et al. 2020), and knowing that E.Coli and Serratia are relatively more susceptible to antibiotics linked to beta lactam mechanisms of action (Waites et al. 2006; Traub 2000) one can conclude that their weak radiation resistance is connected with this matter beside the genetic differences.
Although this strain of E.Coli has shown an insignificant resistance against gamma radiation, its great adapting ability can be explained by genetic adaptation related to DNA repairs [Boiteux et al. 1987; Harris et al. 2009; Byrne et al. 2014] as opposed to Salmonella. Also, its ability to adapt to different harsh environmental conditions such as pollution (Zhang et al. 2019), makes this bacterium another considerable candidate for radiative environment investigations next to Enterobacter Aerogenes that showed the highest adaptability. Other studies have indicated higher resistance of Enterobacter Aerogenes to Gamma radiation (Nei et al. 2012) and mentioned that hydrogen production in its cells is increased after being exposed to this type of radiation (Cheng et al. 2017) which is a considerable reason for its great resistance during the second exposure.