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Email: ruthiran.kugathasan@nhs.net
Introduction
Cycloserine is an alanine analogue with activity against a wide variety of urinary coliforms and has been used previously in some areas of the world for the treatment of urinary tract infections. Trimethoprim is still commonly used as a first line empirical treatment for urinary tract infections despite resistance rates of 35% in the
United Kingdom. The emergence of multi-resistant strains of Enterobacteriaceae and the lack of new agents in the development pipeline has prompted a need to review the activity of older agents.
Susceptibility testing of cycloserine has historically been problematic since standard media contains competitive alanine, leading to falsely elevated minimum inhibitory concentrations (MICs). This study tests urinary coliforms, including resistant phenotypes, against cycloserine in both standard and minimal media.
Materials and Methods
Susceptibilities were performed on 500 “wild type” UTI coliforms against cycloserine, trimethoprim, nitrofurantoin, cefotaxime and ceftazidime using Muller-Hinton broth in the range 0.008-128 mg/l in accordance with ISO guidelines. Cycloserine was also tested in Minimal-Salts media + 2% 1M glucose + 0.2% 1M magnesium sulphate. MICs were recorded after 18 hours incubation at 35⁰C and interpreted with EUCAST breakpoints
(where available).
To exclude differences in growth potential, growth of a 1x10 5 inoculum of Escherichia coli
ACTC 25922 in Mueller-Hinton broth and Minimal-Salts medium following incubation at
35⁰C for 20 hours was estimated by spiral plating onto blood agar and counting resulting colonies .
Results
Table 1: Cycloserine susceptibilities and antimicrobial comparators against all Enterobacteriaceae
Number and type of isolate
MIC
50
(µl/ml)
MIC
90
(µl/ml)
Range %Susc.
%Res.
All isolates
Cycloserine (MS)
Cycloserine (MHB)
Cefotaxime
451
500
500 a b b
2
32
0.06
64
128
0.25
0.12->128
8->128
<0.008->128
N/A
N/A
93.8
N/A
N/A
5.2
Ceftazidime
Nitrofurantoin
Trimethoprim
500 b
500 b
500 b
500 b
0.25
16
1
Fosfomycin
TRM resistant strains
0.25
Cycloserine (MS)
Cycloserine (MHB)
Cefotaxime
Ceftazidime
Nitrofurantoin
182
182 d d
0.25
16
Fosfomycin 182 d 0.25
3rd generation cephalosporin resistant strains
Cycloserine (MS)
Cycloserine (MHB)
21 e
24 f
4
64
Nitrofurantoin
Trimethoprim
Fosfomycin
164 c
182 d
182 d
24 f
24 f
24 f
2
32
0.06
16
>128
0.25
1
64
>128
4
16
128
4
2
64
2
64
>128
128
>128
32
0.03->128
0.5->128
0.06->128
0.12-1024
0.12 - >128
16->128
0.016->128
0.03-128
4 - >128
0.12-128
1->128
16->128
4->128
0.5->128
0.12-64
94.0
93.0
60.6
98.2
N/A
N/A
89.6
90.0
91.0
99.5
N/A
N/A
91.3
21.7
95.7
4.8
7.0
36.4
2.0
N/A
N/A
9.0
9.0
8.8
0.5
N/A
N/A
8.7
69.6
4.3
Cycloserine MIC
50 for the “wild type” coliforms was 32 µg/ml in Mueller-Hinton broth compared to 2 µg/ml in
Minimal-Salts. 87% of “wild type” UTI coliforms show cycloserine MICs <=8 µg/ml in Minimal-Salts. Growth in
Minimal-Salts medium and Mueller-Hinton was similar after 20 hours incubation. Susceptibilities and % resistance to other antimicrobials are shown in Table 1.
Conclusions
The epidemiological cut-off values of cycloserine for E. coli in this study were 64 µg/ml using Mueller-Hinton broth and 8 µg/ml using Minimal-Salts medium. 94% of trimethoprim resistant and 82% of 3 rd generation cephalosporin resistant E.coli had MICs in Minimal-Salts <=8 µg/ml. Escherichia coli accounts for 75-95% of all uncomplicated urinary tract infections and pyelonephritis. Urinary cycloserine concentrations of 50 µg/ml at 8hrs and 30 µg/ml at 24hrs can be achieved following standard dosing of 250mg [1]. Cycloserine toxicity typically occurs at plasma levels above 30 µg/ml [2]. Our data suggests cycloserine could be used at a lower dose and still effectively treat urinary tract infections, while lowering the chance of toxicity. Cycloserine is still licensed in some countries for the treatment of urinary infections and the data presented here suggests that it may have a role in the management of infections resistant to trimethoprim and 3 rd generation cephalosporins.
References
1.
P. Kaltenis (1986). Cycloserine as a urinary tract antiseptic. Institute of urology and nephrology. 1986, Volume 18, Issue 2, pp 125-130.
2.
Maclean, R. L (1956). Transactions of the 15 th conference on the chemotherapy for tuberculosis. Veterans Administration, St. Louis, Missouri.
Kugathasan R 1 , Wootton M, Howe R. (2014). Cycloserine as an alternative urinary tract infection therapy: susceptibilities of 500 urinary pathogens to standard and alternative therapy antimicrobials. Eur J Clin Microbiology Infectious
Disease 2014 Jan 29. [Epub ahead of print]. The final publication is available at http://link.springer.com/article/10.1007/s10096-014-2051-9