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 ::  Introduction
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Year : 1976  |  Volume : 22  |  Issue : 4  |  Page : 171-175

In vitro susceptibility of urinary pothogens to trimetho­prim, nitrofurantoin and trimethoprim-nitrofurantoin combination

1 Department of Pharmacology, Clinical Drug trial Unit, Council of Scientific and Industrial Research (CSIR) and The Department of Medicine, Seth G. S. Medical College, Parel, Bombay-400012., India
2 Department of Pharmacology, Seth G. S. Medical College, Parel, Bombay-400012, India

Correspondence Address:
Pervin Anklesaria
Department of Pharmacology, Seth G. S. Medical College, Parel, Bombay-400012.
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Source of Support: None, Conflict of Interest: None

PMID: 802242

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 :: Abstract 

Trimethoprim and Nitrofurantoin are both effective against urinary tract infections. Trimethoprim till now has been used in combination with sulphamethoxazole. We have investigated the combination of Trimethoprim and Nitrofurantoin on 182 pathogenic cultures isolated from 165 cases of urinary tract infection which had a significant colony count of >10 5 orgs/ml. 65% of the strains exhibited synergy to the combination. Clinical results of this combi­nation are very promising and are reported in another communica­tion.

How to cite this article:
Anklesaria P, Paranjpe S, Gupta K, Acharya V, Paul T, Sheth U K. In vitro susceptibility of urinary pothogens to trimetho­prim, nitrofurantoin and trimethoprim-nitrofurantoin combination. J Postgrad Med 1976;22:171-5

How to cite this URL:
Anklesaria P, Paranjpe S, Gupta K, Acharya V, Paul T, Sheth U K. In vitro susceptibility of urinary pothogens to trimetho­prim, nitrofurantoin and trimethoprim-nitrofurantoin combination. J Postgrad Med [serial online] 1976 [cited 2022 Dec 8];22:171-5. Available from:

 :: Introduction Top

Infection of the urinary tract is one of the most common bacteriological diseases encountered. This condition pro­duces a favourable environment for the continuous multiplication of bacteria in the urinary system. Urine culture is the only satisfactory method of diagnosis of active infection.

Infection of the urinary tract leads to the administration of antimicrobial drugs. The objective of antimicrobial therapy is to permanently eliminate bacteria from the urinary tract, which is determined by quantitative bacterial counts decreasing within 48-72 hours after initiation of therapy. [7] The urinary level of an anti­microbial agent generally determines the outcome of therapy when the infection is unaccompanied by sepsis. [8] Good urinary concentrations are achieved after oral ad­ministration of most of the commonly used antimicrobial agents namely Nitro­furantoin (NF), Trimethoprim (TMP) , ampicillin, cephalexin etc. It is well known that NF produces untoward side effects such as nausea and vomiting in a consi­derable number of patients.

In recent years, as many urinary tract infections show high degree of resistance to sulfonamides and antibiotics, interest has centered on either new antibiotics which have not been much used or com­bination of two drugs acting at different sites on the organism. Successful intro­duction of cotrimoxazole which is a combination of an antifolic agent Trimetho­prim (TMP) and sulfamethoxazole is an example of such strategy. Trimethoprim and sulfonamides both act at different stages in the synthesis of folic acid for the formation of DNA. TMP alone in ade­quate doses also has been reported to be effective in the treatment of urinary tract infection.

In our hospital a number of bacteria isolated from urinary tract infection have been found to be sensitive to Nitrofuran­toin. The practical difficulty has been intolerance to oral administration of Nitrofurantoin. The mechanism of action of NF is not still well defined. We decid­ed to check the effectiveness of a combi­nation of TMP and NF in decreased doses on the isolated organisms from urinary tract infections. Both these drugs are excreted well in urine and if one could find any additive or synergistic effects of the combination, clinically, in cases sensi­tive to these drugs, smaller doses of NF could be used with better tolerance and adequate effectiveness.

 :: Material and Methods Top

182 bacterial strains were isolated from 165 cases of urinary tract infection which had a significant colony count of >10 5 orgs/ml. The bacteria were identified according to Cowan and Steel. [4]

The minimal inhibitory concentrations (MIC) of TMP, NF and the combination of the two drugs in varying ratios were determined for all the strains. The medium used was nutrient agar pH 7.4, plus 5% lysed horse blood. An overnight broth culture of the organisms appro­priately diluted with nutrient broth was used as the inoculum to streak the agar plates containing the antibacterial agent.

A dilution of 1: 1000 of the overnight broth culture was used for E. coli, Staphy­lococci, A. anitratus and A. faecalis. For Proteus, Pseudomonas, Morganella, Kleb­siella and Enterobacter strains a dilution of 1:10,000 and for Streptococci a dilu­tion of 1:10 was used.

 :: Results Top

The organisms isolated from 165 cases of urinary tract infection were 175 Gram negative bacteria and 7 Gram positive bacteria, namely  Escherichia More Details coli (98),  Enterobacter aerogenes Scientific Name Search , Acinetobac­ter anitratus (7), Alcaligenes faecalis Scientific Name Search  (6), Klebsiella pneumoniae Scientific Name Search  (4), Alkales­cens-dispar group (1),  Pseudomonas aeruginosa Scientific Name Search ), Morganella morganii Scientific Name Search  (1),  Proteus mirabilis Scientific Name Search  (7),  Proteus rettgeri Scientific Name Search (1),  Proteus inconstans Scientific Name Search s (3), mixed cul­ture of Enterobacter + Proteus (3), un­identified Gram negative bacilli (1), Staphylococcus aureus Scientific Name Search  (3), Staphylococ­cus epidermidis (2) and  Streptococcus pyogenes Scientific Name Search r>
The MICs of TMP and NF were determined for the 182 urinary pathogens of which 114 strains (62.6%) had an MIC of < 1µg/ml of TMP and 121 strains (66%) had an MIC of < 100 µg/ml of NF [Figure 1] and [Figure 2].

Out of the 182 bacterial species, 120 strains (65%) demonstrated synergy when tested with varying ratios of the combination of TMP and NF. The results of these 120 susceptible strains are pre­sented 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, teas 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 the 120 strains by the FIC index being < 1.0, the lower the index, the greater the synergy. [5]

The FIC indices varied from 0.13 (1 strain of E. coli) to 0.9 (2 strains of A. faecalis). 26 strains had FIC indices < 0.5; 41 strains between 0.5 to 0.7 and 53 strains had an FIC index of 0.7 to 0.9 [Table 2].

Of the 62 strains that did not exhibit synergy, 20 were E. coli, 23 -E. aerogenes, 5-A. anitratus, 1 -A. faecalis, 4 -K. pneunaoniae, 3 -P. aeruginosa, 1 -Mor­ganella morganii, 2 -P. mirabilis, 1 -En­terobacter + Proteus, and 2 -Staphylo­coccus aureus.

The above 182 strains were also tested by the paper disc method [2] against all the common antibiotics and chemotherapeutic agents as well as the combination of 1.25 µg TMP plus 25 µg NF per disc.

 :: Discussion Top

Enhancement of antibacterial activity of chemotherapeutic agents is desired, especially in the treatment of urinary tract infection. The use of a combination of antimicrobial agents for the manage­ment of severe infections is common in modern practice.

In our in vitro study, we have found that a combination of TMP and NF is useful in inhibiting bacterial strains at low concentrations as compared to the in­dividual drugs when acting alone. A significant reduction in the MIC of NF when combined with TMP is evident [Table 1].

The MIC of TMP of the various uri­nary pathogens is comparable with those reported by Acar et al [1] in their study of urinary tract infections.

Bushby [3] stated that synergy is maximal when the organisms are susceptible to both drugs. This was not the case in our study where we found that, though the bacteria were susceptible to both TMP and NF, the FIC indices varied from 0.13 to 0.8 showing a wide range synergy. Our findings show that strains resistant to TMP and sensitive to NF, exhibit maximum synergy with the exception of 2 strains of E. coli and those resistant to both drugs, demonstrate moderate degree of potentiation except 3 strains of A. f aecalis, 1 strain of A-D group and 2 strains of P. aeruginosa which showed minimal synergy.

TMP acts by inhibiting the enzyme di­hydrofolate reductase of the bacterial cell. [6] Since the action of NF is not known, the synergistic action of the combination cannot be explained.

As the drugs were not combined in the optimal ratio of their MICs, the ex­cess of one drug reduced the MIC of the other drug considerably below that of the optimal ratio. This finding is supported by Bushby. [3]

Clinical trials with this combination have shown promising results and are reported in another communication.

 :: References Top

1.Acar, J. F., Goldstein, F. and Chabbert, Y. A.: Synergistic Activity of Trimetho­prim-Sulfamethoxazole on. Gram-negative Bacilli. Observations In Vitro and In Vivo. J. Infec. Dis., 128 (Suppl.): 8470-5477, 1973.  Back to cited text no. 1    
2.Bauer, A. W., Kirby, W. H., Sherris, J. C. and Turk, M.: Antibiotic susceptibility testing by a standardised single disc me­thod. Amer. J. Clin. Path., 45: 493-496, 1966.  Back to cited text no. 2    
3.Bushby, S. R. M.: Trimethoprim-Sulfa­methoxazole: In Vitro microbiological Aspects. J. Infect. Dis., 128 (Suppl.): S442-S462, 1973.  Back to cited text no. 3    
4.Cowan and Steel's Manual for the Identi­fication of Medical Bacteria. 2nd Ed., Cambridge University Press, 1974.  Back to cited text no. 4    
5.Elion, G. B., Singer, S. and Hitchings, G. H.: Antagonists of nucleic acid deri­vatives. J. Biol. Chem., 208: 477-488, 1954.  Back to cited text no. 5    
6.Hitchings, G. H.: Species differences among dihydrofolate reductases as a basis for chemotherapy. Postgrad, Med. J. 45 (Suppl.) : 7-10, 1969.  Back to cited text no. 6    
7.Kaye Donald: Urinary Tract Infection and its Management. The C. V. Mosby Co., St. Louis, p. 188, 1972.  Back to cited text no. 7    
8.Stamey, T. A.: Urinary Infections. Wil­liams and Wilkins, Baltimore, p. 33, 1973.  Back to cited text no. 8    


  [Figure 1], [Figure 2]

  [Table 1], [Table 2]


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