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

Abstract

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 dysentriae 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 inoculum of Salmonella typhi,Shigella dysentriae, Proteus mirabilis and Staphylococcus aureus were prepared, each and was streaked onto Mueller-Hinton Agar plates. Prepared Amoxicillin, Ciprofloxacin, and Gentamicin paper discs were placed each at the center of the plates and incubated for 24 hours, at 37oC. 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 on to fresh Mueller- Hinton Agar plates. This process was repeated of 10 timesfor each. Over the course of 10 exposures to test antibiotics separately, all the test organisms developed resistance to the antibiotics gradually as seen through decrease in diameter of their zones of inhibitions. Salmonella typhi plates (from 1-10 exposure) under Ciprofloxacin, on average were reduced to 24.1mm, Gentamycin were reduced to 6.5mm and Amoxicillin were reduced to 5.8mm. Shigella dysentriae plates (from 1-10 exposure) under Ciprofloxacin, on average were reduced to 27.2mm, Gentamycin were reduced to 7.8mm and Amoxicillin were reduced to 6.0mm. Proteus mirabilis plates (from 1-10 exposure) under Ciprofloxacin, on average were reduced to 34.0mm, Gentamycin were reduced to 22.7mm and Amoxicillin were reduced to 8.5mm. Staphylococcus aureus plates (from 1-10 exposure) under Ciprofloxacin, on average were reduced to 25.9mm, Gentamycin were reduced to 15.4mm and Amoxicillin were 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 has notable zone of inhibition often repeated exposure while Amoxicillin was the least active antibiotic as it showed full resistance at 4th exposure for Salmonella typhi and Shigella dysenteriae and 5th exposure for Proteus mirabilis and Staphylococcus aureus.