**1. Introduction**

Infectious diseases caused by virus, bacteria and fungi represent a major apprehension globally in terms of detrimental public health and economy. Some of the infectious agents such as virus (e.g., Coronavirus, influenza, Ebola, chickenpox), bacteria (e.g., tuberculosis, cholera, whooping cough) are highly contagious and are responsible for communicable diseases. Communicable diseases spread from person to person through close contact including touching and kissing, also via coughing and sneezing, contamination of food and water. Many bacteria are also opportunistic pathogens and are commonly cause mild–moderate infections including sore throat, acne, tooth decay, urinary tract infections, cellulitis/skin infection, sexually transmitted infection, bacterial vaginosis, peptic/stomach ulcer, keratitis/eye infection, to severe/life-threatening infections such as pneumoniae, septicaemia/sepsis, meningitides in humans, animals, and birds. Most opportunistic bacteria exist as a commensal flora within the host body (gastrointestinal tract, skin, mucosal, oral, and nasal cavity, urogenital tract) and commonly found in abiotic surfaces (water, food, soil) in the environment [1, 2]. Under normal conditions i.e., in healthy people these bacterial pathogens do not cause infections. Infections caused by opportunistic bacteria are primarily triggered by either invasion of host commensal bacteria or bacteria from environmental sources gets into host bodily tissue [1, 2]. However, these opportunistic bacteria primarily target and cause fatal infections in immunocompromised people including acquired immunodeficiency syndrome (AIDS/HIV positive) patients, cancer patients (treated with immunosuppressive drugs, corticosteroids), hospital admitted patients for surgery, patients with underlying diseases such as cystic fibrosis, diabetes [2]. Most common examples of opportunistic bacteria found in mammals, birds and environment are *Pseudomonas aeruginosa*, *Staphylococcus aureus*, *Staphylococcus epidermidis*, *Escherichia coli, Klebsiella pneumoniae, Neisseria meningitides, Acinetobacter baumannii, Helicobacter pylori*, *Streptococcus pneumoniae*, *Streptococcus pyogenes*, *Streptococcus mutans*, *Clostridioides difficile, Legionella pneumophila, Propionibacterium acnes*, etc. On the other hand, mammalian and bird's body also host different species of good bacterial species (probiotics) which are essential for general wellbeing [3]. Lactobacillus and Bifidobacteria species are the common example of probiotics bacteria that are present in the mammalian digestive tract, they aid in maintaining daily healthy lifestyle includes food digestion, balancing pH of the body, alleviate symptoms of Gastroesophageal reflux diseases (GERD) such as heartburn, acid-reflux [4, 5].
