Oral fosfomycin activity against Klebsiella pneumoniae in a dynamic bladder infection in vitro model

Abstract

Introduction: The use of oral fosfomycin for urinary tract infections (UTIs) caused by non-Escherichia coli uropathogens is uncertain, including Klebsiella pneumoniae, the second most common uropathogen.

Methods: A multicompartment bladder infection in vitro model was used with standard media and synthetic human urine (SHU) to simulate urinary fosfomycin exposure after a single 3 g oral dose (fAUC0-72 16884 mg·h/L, t½ 5.5 h) against 15 K. pneumoniae isolates including ATCC 13883 (MIC 2 to >1024 mg/L) with a constant media inflow (20 mL/h) and 4-hourly voiding of each bladder. The impact of the media (CAMHB + G6P versus SHU) on fosfomycin MIC measurements, drug-free growth kinetics and regrowth after fosfomycin administration was assessed. A low and high starting inoculum (5.5 versus 7.5 log10 cfu/mL) was assessed in the bladder infection model.

Results: Compared with CAMHB, isolates in SHU had a slower growth rate doubling time (37.7 versus 24.1 min) and reduced growth capacity (9.0 ± 0.3 versus 9.4 ± 0.3 log10 cfu/mL), which was further restricted with increased inflow rate (40 mL/h) and more frequent voids (2-hourly). Regrowth was commonly observed in both media with emergence of fosfomycin resistance promoted by a high starting inoculum in CAMHB (MIC rise to ≥1024 mg/L in 13/14 isolates). Resistance was rarely detected in SHU, even with a high starting inoculum (MIC rise to ≥1024 mg/L in 2/14 isolates).

Conclusions: Simulated in an in vitro UTI model, the regrowth of K. pneumoniae urinary isolates was inadequately suppressed following oral fosfomycin therapy. Efficacy was further reduced by a high starting inoculum.

Figure 1.

Fosfomycin MIC distribution among clinical K. pneumoniae urinary isolates (n = 50). MIC performed by agar dilution. Solid bars represent isolates with MICs within the WT range.

Figure 2.

Drug-free growth under static and dynamic incubation conditions. Bacterial growth is presented as the average (±SD) of the 14 clinical isolates and the ATCC strain. Static incubation performed in 20 mL media, incubated at 36°C with vigorous shaking (200 rpm), without media inflow or outflow. Dynamic incubation performed in the bladder infection in vitro model under two different conditions: media inflow at 20 mL/h with 4-hourly voids compared with 40 mL/h with 2-hourly voids. The grey shaded area highlights the time over which the growth rate was assessed (2–4 h). This figure appears in colour in the online version of JAC and in black and white in the print version of JAC.

Figure 3.

Growth response after a simulated single 3 g dose of oral fosfomycin. Each experiment is represented by two graphs illustrating the total growth (quantitative growth on drug-free MHA) and the fosfomycin-resistant growth (quantitative growth on MHA supplemented with 512 mg/L fosfomycin and 25 mg/L G6P). (a) Testing in CAMHB with 25 mg/L G6P with a low starting inoculum. (b) Testing in CAMHB with 25 mg/L G6P with a high starting inoculum. (c) Testing in SHU with a low starting inoculum. (d) Testing in SHU with a high starting inoculum. Solid lines represent the total growth. Dashed lines represent fosfomycin resistant growth. The 15 isolates are grouped by their baseline fosfomycin MIC measurement (2–4, 8, 16–32 and >32 mg/L). ATCC 13883 was run in duplicate. This figure appears in colour in the online version of JAC and in black and white in the print version of JAC.