In vitro Induction of Phenotypic Resistance to Antibiotics in some Pathogenic Bacteria

The study was carried out on the development of resistance due to repeated exposure of some bacterial isolates to antibiotics. The organisms; Salmonella typhi and Shigella dysenteriae were isolated from stools using Salmonella Shigella Agar (SS Agar). Proteus mirabilis and Staphylococcus aureus were isolated from urine using Cystine Lysine Electrolite Deficient (CLED) Agar. After the isolation, standard inoculums of Salmonella typhi, Shigella dysenteriae, Proteus mirabilis, and Staphylococcus aureus were prepared, each and streaked onto Mueller-Hinton Agar plates. Prepared Amoxicillin, Ciprofloxacin, and Gentamicin paper discs were placed at the center of the plates and incubated for 24 hours at 37°C. Zones of inhibition were formed. The zones of inhibition were measured and recorded, and then bacteria from the edges of the inhibition zones were picked up with a swab stick and inoculated onto fresh Mueller-Hinton Agar plates. This process was repeated 10 times for each. Over the course of 10 exposures to test antibiotics separately, all the test organisms developed resistance to the antibiotics gradually, as seen through a decrease in the diameter of their zones of inhibition. Salmonella typhi plates (from 1-10 exposures) under Ciprofloxacin, on average, were reduced to 24.1mm, Gentamicin was reduced to 6.5mm, and Amoxicillin was reduced to 5.8mm. Shigella dysenteriae plates (from 1-10 exposures) under Ciprofloxacin, on average, were reduced to 27.2mm, Gentamicin was reduced to 7.8mm, and Amoxicillin was reduced to 6.0mm. Proteus mirabilis plates (from 1-10 exposures) under Ciprofloxacin, on average, were reduced to 34.0mm, Gentamicin was reduced to 22.7mm, and Amoxicillin was reduced to 8.5mm. Staphylococcus aureus plates (from 1-10 exposures) under Ciprofloxacin, on average, were reduced to 25.9mm, Gentamicin was reduced to 15.4mm, and Amoxicillin was reduced to 7.4mm. The results obtained confirmed that repeated exposure of the bacterial pathogens to antibiotics increased their resistance. Ciprofloxacin was the most active antibiotic among the test antibiotics, as it had notable zones of inhibition even after repeated exposure, while Amoxicillin was the least active antibiotic, as it showed full resistance at the 4th exposure for Salmonella typhi and Shigella dysenteriae and the 5th exposure for Proteus mirabilis and Staphylococcus aureus.