In vitro susceptibility of urinary pathogens to trimethoprim, nitrofurantoin and trimethoprim-nitrofurantoin combinationPramila Patil, Vidya Sharma, BN Joshi
Department of Microbiology, B.J.Medical College, Poona-1, India
Correspondence Address: Source of Support: None, Conflict of Interest: None PMID: 119047
Source of Support: None, Conflict of Interest: None
A combination of trimethoprim and nitrofurantoin was tried against 100 bacterial strains isolated from cases of urinary tract infection. The combinations in varying ratios have shown a good synergistic effect against the common urinary pathogens. However, the results will have to be corroborated by conducting trials in a large number of patients.
Urinary tract infection is one of the commonest problems faced by the clinicians. Quite often, it is a ver3 frustating experience. A large number of antibiotics are available for the clinicians but more effective ones have, many times, some or other untoward effect, limiting their frequent use. Attempt; have been made, therefore, to lessen the doses of such antimicrobial drugs by using them in combination with some other singly effective drug. One such recent example has been the use of cotrimoxazole combining trimethoprim (TMP) ant sulphamethoxazole.
In our laboratory, a number of bacteria isolated from the urinary tract are found to be sensitive to nitrofurantoin (NF) After oral administration of nitrofurantoin, a good urinary concentration of the drug is reached. However, this involves a practical difficulty of intolerance to suet high doses. An attempt is, therefore, made to lessen the dose of nitrofurantoin by combining it with another equally effective drug namely trimethoprim (TMP), also in lesser doses. The present communication reports the preliminary findings obtained in our laboratory by using a combination of TMP + NF in varying concentrations.
One hundred bacterial strains were isolated from cases of urinary tract infection having a significant colony count of 10 organisms/ml. The bacterial identification was done by usual methods. 
The medium used for isolation of organisms was nutrient agar incorporating 5% lysed horse blood. An overnight broth culture of the organisms appropriately diluted with nutrient broth and inoculum was used to streak the agar plates containing the antibacterial agents in varying ratios for determining minimum inhibitory concentration (MIC) of TMP and NF.
A dilution of 1: 1000 of the overnight broth culture was used for E. coli and Staphylococci. For Proteus, Pseudomonas, Klebsiella and Enterobacter strains, a dilution of 1: 10,000 and for Streptococci a dilution of 1: 10 was used.
The organisms isolated from the urine samples contained 95 Gram negative bacilli and 7 Gram positive cocci, namely E. Coli Sp. (51), Klebsiella. Sp. (24), Proteus Sp. (6) Pseudomonas aerugenosa (12), Staphylococcus aureus (5) and Streptococcus pyogenes r>
The MICs of TMP and NF were determined for the 100 urinary pathogens, 67 strains had an MIC of < 1 µg/ml of TMP and 81 strains had an MIC of < 100 1Lgjmi of NF.
Out of 100 bacterial species, 43 strains demonstrated synergy when tested with varying ratios of the combination of TMP and NF. The results are shown in [Table 1].
The fractional inhibitory concentration (FIC) of each drug, which is defined as the MIC of the drug in combination divided by the MIC of the drug alone, was calculated for the individual strains. The sum of the FICs of TMP and NF for each organism gave the FIC index. Potentiation by the combination was demonstrated for 43 strains by the FIC index being < 1.0. The lower the index, the greater the synergy.
The FIC indices varied from 0.13 to 0.9. Four strains had FIC indices < 0.5; 20 strains between 0.5 to 0.7 and 19 strains had an FIC index of 0.7 to 0.9 [Table 2].
All the strains were also tested by the paper disc method against Cotrimoxazole as well as the combination of 1.25 µg of TMP plus 25 µg of NF per disc. Results are equivocal in most strains except for E. coli where promising results are seen [Table 3].
In our laboratory a combination of TMP and NF was found useful in inhibiting bacterial strains at lower concentration, as compared to the individual drugs acting alone. This effect was found in about 60%, of E. coli strains. Considering that E. coli is the commonest organism causing chronic urinary tract infection, it is oboious that the combination of NF and TMP in smaller doses is going to be of immense help in treating such cases. The synergistic effect was also evident in about 33% cases of Proteus infection.
The combination was most pronounced against Gram positive Cocci though the number of strains tested is small. Anklesaria et al  have also reported a good synergistic effect of this combination against E. coli. Acar et al  also report a good synergistic effect against this common urinary pathogen. In our study, similar to Anklesaria et al,  the bacteria were found susceptible to both TMP and NF and the FIC indices varied from 0.13 to 0.9 showing a wide range though most of our strains fell in the range 0.5-0.9. Our in vitro studies therefore, using a combination of TMP and NF in varying ratios have shown a good synergistic effect against, the common urinary pathogens namely E. coli, Proteus and Gram positive cocci.
These results will have to be corroborated by trying such combinations in patients with urinary tract infections.
[Table 1], [Table 2], [Table 3]