Antibiotic Discovery, Bacterial Resistance, and Gene Transfer: Assessing Impact and Versatility in E. coli Strains

In 1928, Alexander Fleming discovered antibiotics, marking a crucial milestone in treating bacterial infections. Over time, bacteria naturally developed resistance, which can be transferred between different species through horizontal gene transfer. Genes for resistance are carried on plasmids or transposons, transferred through transformation, transduction, or conjugation. These genes have diverse applications in agriculture and medicine, addressing pathogenicity and improving antibiotic medications.

The lab aims to assess bacterial resistance to specific antibiotics and observe gene transfer by mating E. coli strains for potential offspring with Ampicillin and Chloramphenicol resistance.

E. coli cultures at different times are plated on agar with antibiotics, and organisms with both resistances are selected. Longer incubation is expected to yield more growth.

The second part investigates bacterial susceptibility to antibiotics. It is hypothesized that cell wall-affecting antibiotics are more harmful to gram-positive bacteria, while protein activity-affecting antibiotics impact both types equally.

Methods involve conjugative gene transfer and antibiotic sensitivity tests. Observations, delayed by a day due to a long weekend, affected bacterial count and zone of inhibition, as prolonged incubation led to increased growth.

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Conjugative Transfer of Antibiotic Resistance Genes between E.coli Strains: During this phase of the experiment, the transfer rate of plasmids between E.coli strains was investigated by monitoring the growth of E.coli colonies. Table 1.1 indicates that the E.coli sample incubated for 40 minutes exhibited the highest growth, while the sample incubated for 10 minutes showed the least. The 10-1 dilution had the most growth, and no growth was observed in the 10-4 dilution. The 10-minute incubation sample showed growth in the 10-1 and 10-2 dilutions, but the colony count in the 10-2 dilution was unreliable (only 3 colonies).

The 20-minute sample produced viable results in the 10-2 and 10-3 dilutions, while the 30-minute sample showed viable growth in the first three dilutions. The sample incubated for 40 minutes displayed the most growth, but viable results were only obtained in the 10-3 dilutions. These results were converted to cfu/mL (Table 1.2) using the formula: colonies / (amount plated * dilution factor). The cfu/mL pattern mirrored that of Table 1.1, confirming maximal growth in the 40-minute E.coli sample and minimal growth in the 10-minute sample.

Graph 1.1 and Graph 1.2 illustrate an exponential growth of E.coli bacteria over time, with a steady increase after 30 minutes.

Sensitivity of Four Microorganisms to Eight Antibiotics: In this segment, four bacterial strains (E.coli, M. Luteus, P.Fluorescens, and S.cerevisiae) were exposed to eight antibiotics to assess their resistance. The resistance was measured by determining the zone of inhibition, the clearing area around each bacterial strain. E.coli exhibited the largest inhibition zone with Chloramphenicol, while no inhibition zone was observed with Penicillin. M. Luteus displayed the least resistance to Ampicillin and the most resistance to Neomycin. P.Fluorescens had the largest zone of inhibition around Tetracycline but showed high resistance to Chloramphenicol and Penicillin. S.cerevisiae was only susceptible to Neomycin.

The observed pattern aligned with the hypothesis that gram-negative bacteria would be more resistant to antibiotics affecting cell wall synthesis, while antibiotics impacting protein activity would affect both bacterial types equally. The results indicated that the chosen antibiotics had varying effects on different bacterial strains, with Tetracycline exhibiting versatility by affecting protein activity, making it effective against both gram-positive and gram-negative bacteria.