# _Pseudomonas aeruginosa_ Response to Acidic Stress and Imipenem Resistance
## 1 Introduction  
_Pseudomonas aeruginosa_ (_P. aeruginosa_) is a Gram-negative opportunistic pathogen responsible for nosocomial, community-acquired, and food borne infections [1, 2]. _P. aeruginosa_ is well known for its adaptability to diverse environments, antibiotic resistance, and production of extracellular and cell-associated virulence factors [2, 3]. Most often, _P. aeruginosa_ has been associated with hospital-acquired infection; however, it is one of the major causes of food spoilage and foodborne diseases [4, 5]. It has a tendency to grow and survive in a wide range of pHs (4.5 to 9.0) and a high saline environment [6]. Therefore, it is regularly reported in different food items, e.g., vegetables and fruits, dairy products, beef, mutton,fish products, etc. _P. aeruginosa_ also colonizes a wide variety of surfaces and food packaging materials [7; 8].  
Pseudomonas possesses a strong tendency to develop biofilm, which aids in its persistence on different surfaces [9]. Biofilm formation ability is directly related to the lethality of the organism and offers resistance to the host immune system, chemotherapy, and disinfectants [10]. The biofilm matrix supports the anchoring of these cells to various surfaces and helps them survive in hostile and unfavorable environmental conditions [6; 11]. The matrix generally comprises extracellular metabolites, including polysaccharides, environmental DNA, proteins, lipids, and dead cells [12; 13]. The composition of this matrix may vary for different environments and surfaces [14]. The major components of biofilm consortia are polysaccharides, which initiate attachment to the surface and cement the surrounding cells together. Furthermore, it has been reported that biofilm consortia harbor different types of cells, including dead bacterial cells, and metabolically active and inactive cells [15]. Planktonic or metabolically active populations are gradually released or dispersed from these consortia, to colonize new surfaces [16; 17].  
_P. aeruginosa_ shifts between different lifestyles, to survive in toxic and unfavorable conditions [13; 18]. It has been reported that _P. aeruginosa_ biofilms harbor small colony variants, rugose small-colony variants, and persister cells [19; 20]. Metabolically inactive populations consist of small colony variants, persister cells, and viable but non-cultivable cells. These phenotypes, with slow metabolic rates, have the potential to tolerate and survive different classes of antimicrobial agents [15].  
A study conducted by [PERSON] and collaborators demonstrated that acidic pH is one of the major factors triggering biofilm formation in _P. aeruginosa_[21]. Accumulation of eDNA in biofilms and infection sites can acidify the local environment, which further promotes the growth of antibiotic-resistant phenotypes of _P. aeruginosa_. Moreover, low pH may in itself be the factor responsible for the formation of metabolically inactive phenotypes or slow-growing antibiotic-resistant variants having the potential to revert to the wild-type virulent population.  
The present study unearthed novel aspects of phenotypic switching of _P. aeruginosa_ in the acidic environment of pulpy juices and under the stress of an antibacterial agent (imipenem), by triggering biofilm formation, consisting of slow growing resistant variants and normal-pigmented phenotypes. The subject isolates showed heterosexual to imipenem. Slow growing variants outnumbered the normal population under imipenem stress in the adherent biofilms. This phenomenon was found to be reversible when the stress was relieved and cells were supplemented with sufficient nutrients.