In Vitro Activity of Fosfomycin and Nitrofurantoin Against Contemporary Enterobacterales Pathogens Isolated from Indian Tertiary Care Hospitals

Abstract

Background:

In India, multidrug resistance in community and hospital associated Gram-negative pathogens has increased sharply over the past few years. In the absence of novel oral multidrug resistant-pathogen active therapies, the therapeutic situation with regard to community infections is even more challenging. Hence, the focus is now shifting toward potentially expanding the utility of older antibiotics such as fosfomycin and nitrofurantoin beyond their approved pathogen coverage. The current study was undertaken to assess the activity of fosfomycin and nitrofurantoin against Enterobacterales pathogens through minimum inhibitory concentration (MIC) determination to facilitate monitoring future shifts in susceptibility to these agents.

Materials and Methods:

The present study used 1,350 Enterobacterales, recently collected from various Indian tertiary care hospitals and preserved at Wockhardt Strain Repository. The MIC_50/90 for fosfomycin and nitrofurantoin and the comparator antibiotics was determined for Escherichia coli (N = 470), Klebsiella pneumoniae (N = 429), Enterobacter spp., (N = 144), Proteus spp. (N = 262), and Citrobacter spp. (N = 45), using Clinical and Laboratory Standards Institute recommended agar dilution method.

Results:

Applying E. coli breakpoints, the susceptibility rates of fosfomycin for E. coli, K. pneumoniae, Enterobacter spp., Proteus spp., and Citrobacter spp., were 95.5%, 53.2%, 71.5%, 76.7%, and 91.1%, respectively. Applying respective breakpoints, the susceptibility rates of comparator drugs, including meropenem, were lower than fosfomycin. Susceptibility of nitrofurantoin for E. coli and Citrobacter isolate was 83%, while limited coverage (<13.2% susceptibility) was observed for other genera.

Conclusion:

Amidst widespread resistance, a > 70% fosfomycin susceptibility observed for clinical isolates, including strains expressing carbapenemases, is encouraging and supports conducting additional susceptibility and pharmacokinetic/pharmacodynamic studies to explore its potential for expanded therapeutic use. Nitrofurantoin activity spectrum was restricted to E. coli and Citrobacter spp. and, therefore, offers a relatively limited therapeutic scope.

Introduction

The dissemination of multidrug resistance among Gram negatives poses a significant therapeutic challenge. India represents a hot spot of Gram-negative resistance, which is evident from a recent surveillance study showing extremely high resistance rates to mainstay Gram-negative therapies such as fluoroquinolones (76%) and cephalosporins (77%)^1,2 among hospital pathogens. Disturbingly, the resistance to carbapenem drugs, which are viewed as last resort therapies, is also alarmingly high in India (18–27%).¹ In hospitals, such a scenario compels clinicians to deploy safety/efficacy-compromised drugs such as colistin and tigecycline for the treatment of infections caused by carbapenem-resistant Pseudomonas aeruginosa, Enterobacterales, and Acinetobacter baumannii.^3,4

The situation in the management of community infections is even more alarming as there is a severe dearth of oral therapies active against contemporary Gram-negative pathogens. As a result, successful management of certain common infections such as urinary tract infection (UTI) or urogenital infections in the community has almost turned impossible. Lack of clinically efficacious drugs commensurate to community use necessitates hospitalization exposing the patient to the risk of nosocomial infections and also leads to steep escalation in treatment cost. With the added complexity of lack of information on the identity and the susceptibility profile of pathogens in community settings, clinicians face tremendous therapeutic dilemmas. Under such situations, the interest in the older oral antibiotics such as fosfomycin and nitrofurantoin has rekindled; however, the FDA label mandates to limit the use of these agents only for the treatment of Escherichia coli infections. Since community infections often involve members of genus Klebsiella, Enterobacter, Proteus, and Citrobacter,⁵ it is prudent to assess the prevailing susceptibility rates of fosfomycin and nitrofurantoin for such pathogens. Such information would possibly help judge the potential therapeutic utility of these drugs for the treatment of infections caused by non E. coli pathogens.

Fosfomycin tromethamine and nitrofurantoin are orally absorbable broad-spectrum antibiotics used to treat uncomplicated UTIs (acute cystitis), excluding pyelonephritis, perinephric abscess, or UTI associated with bacteremia caused by E. coli or Enterococcus faecalis.^6–8 Fosfomycin and nitrofurantoin are known to overcome multidrug resistance in Gram negatives owing to their unique mechanism of action not shared by other classes of antibiotics. Moreover, features such as high urinary and adequate safety profile associated with these drugs potentially confer interesting therapeutic prospects even for infections caused by resistant pathogens.⁹ As a result, fosfomycin and nitrofurantoin have gained the status of “salvage therapies” in high resistance community settings in countries such as India.¹⁰

The present study was undertaken to assess the current level of activities of fosfomycin and nitrofurantoin against contemporary Gram-negative Enterobacterales to help judge their use for empiric treatment of Gram-negative infections in community settings. The study would also provide baseline susceptibility rates and help monitor the future shifts in pathogen susceptibilities, if any.

Materials and Methods

Clinical isolates

The article does not include any clinical trial-related data and therefore IRB approval-related requirement is not applicable.

The present study utilized 1,350 clinical isolates comprising of pathogens commonly involved in uncomplicated and complicated UTIs such as E. coli (N = 470), Klebsiella pneumoniae (N = 429), Enterobacter spp. (N = 144), Proteus spp. (N = 262), and Citrobacter spp. (N = 45). These isolates were part of Wockhardt Strain Repository. The isolates were recently collected from several tertiary care hospitals across India located in geographically distinct seven states of the country. The inclusion criteria were one isolate per patient who was hospitalized for minimum 2 days in the hospital departments such as surgery, medicines, intensive care units (ICUs), and gynecology. Bacterial species were confirmed using MALDI-TOF-based VITEK MS (bioMérieux). Cultures were revived and passaged twice in tryptone soya agar (HiMedia, India) medium before minimum inhibitory concentration (MIC) determination.

Susceptibility testing

MICs of fosfomycin (Sigma) and nitrofurantoin (Sigma) were undertaken using agar dilution method following Clinical and Laboratory Standards Institute (CLSI, M07, A9) guidelines.¹¹ MIC of fosfomycin was determined by supplementing 25 mg/L of glucose-6-phosphate (Sigma) in Mueller Hinton agar. MICs were also determined for comparator antibiotics such as piperacillin–tazobactam (PIP-TAZ), ceftazidime–avibactam (CAZ-AVI), trimethoprim–sulfamethoxazole (SXT), ceftriaxone, CAZ, ciprofloxacin, amikacin, and meropenem (MEM) as per CLSI recommended agar dilution method. Comparator antibiotics were recovered from their respective commercial preparations, and purity was ascertained by high performance liquid chromatography analyses conducted at Wockhardt Research Centre. Phenotypic resistance mechanisms potentially involving the presence of β-lactamases such as Extended Spectrum Beta-Lactamase (ESBL), class C, ESBL+class C, and Metallo-Beta-Lactamase (MBL) were identified as per CLSI guidelines (CLSI M100, 30th ed).¹² For phenotypic identification of strains producing OXA-48/181 like β-lactamases, MICs of CAZ in combination with AVI and MICs of imipenem in combination with relebactam were used. The strains were categorized as OXA-48/181 like β-lactamase producers by showing a reduction in CAZ MICs in presence of 4 mg/L of AVI along with no reduction in imipenem MICs in presence of relebactam (+4 mg/L).^13,14 This is because, unlike relebactam, AVI is a good inhibitor of OXA-48/181 like β-lactamases and, therefore, restores the activity of CAZ against such strains.

Interpretation of susceptibility results

Percent susceptibility of isolates to fosfomycin and nitrofurantoin was determined using CLSI breakpoints. For fosfomycin, CLSI recommended MIC interpretive criteria (S ≤ 64 mg/L, I = 128 mg/L, R ≥ 256 mg/L) for E. coli were used for all the Enterobacterales pathogen groups. Similarly, the MIC of nitrofurantoin was interpreted based on CLSI recommended criteria for E. coli (S ≤ 32 mg/L, I = 64 mg/L, R ≥ 128 mg/L). Percent susceptibility of isolates to other drugs was interpreted based on respective CLSI breakpoints.

Results

A total of 1,350 Gram-negative clinical isolates collected during January 2016 to June 2018 from 16 Indian tertiary care hospitals were subjected to MIC determination. The MIC_50/90 of fosfomycin for E. coli was observed to be 2/64 mg/L, whereas the MIC_50/90 values against K. pneumoniae, Enterobacter spp., Proteus spp., and Citrobacter spp. were 64/>256, 64/>256, 32/256, and 2/64 mg/L, respectively (Table 1). Applying CLSI susceptibility breakpoints, E. coli showed high susceptibility rates (95.5%) for fosfomycin, while susceptibility rates of E. coli for PIP-TAZ, CAZ-AVI, SXT, ceftriaxone, CAZ, ciprofloxacin, amikacin, and MEM were 56.3%, 79.8%, 31.6%, 17.3%, 24.5%, 8.0%, 72.3%, and 77.6%, respectively. For K. pneumoniae (n = 429), fosfomycin MIC_50/90 was 64/>256 mg/L, while MIC₉₀ of PIP-TAZ, CAZ-AVI, SXT, ceftriaxone, CAZ, levofloxacin, amikacin, and MEM was >256, 128, >256, >256, >256, 64, >256, and 128 mg/L, respectively. Thus, using E. coli breakpoints, moderate susceptibility rate of fosfomycin (53.1%) toward K. pneumoniae was noted, while for other comparator antibiotics such as PIP-TAZ, CAZ-AVI, SXT, ceftriaxone, CAZ, levofloxacin, amikacin, and MEM, susceptibility to K. pneumoniae ranged from 11.8% to 64.2%. Enterobacter spp., which is also known to be involved in complicated urinary tract infection (cUTI), demonstrated higher susceptibility to fosfomycin (71.5%) compared to PIP-TAZ, CAZ-AVI, SXT, ceftriaxone, CAZ, ciprofloxacin, amikacin, and MEM. In contrast, the Proteus spp., showed higher susceptibility toward fosfomycin, PIP-TAZ, CAZ-AVI, and MEM (≥75%). Similar to the observation with E. coli, Citrobacter spp., too showed excellent susceptibility (91.1%) to fosfomycin, while the susceptibility rates to PIP-TAZ, CAZ-AVI, amikacin, and MEM were 78.7%, 85.1%, 74.5%, and 89.4%, respectively (Table 2). Thus, taking into account all the Enterobacterales studied, fosfomycin retained better activity compared to PIP-TAZ, CAZ-AVI, SXT, ciprofloxacin, amikacin, and MEM.

Table 1.

Summary of the In Vitro Activity of Fosfomycin and Nitrofurantoin Against Gram-Negative Clinical Isolates

No. of isolates inhibited by fosfomycin and nitrofurantoin [MIC (μg/mL)]															MIC₅₀	MIC₇₅	MIC₉₀
	0.06	0.12	0.25	0.5	1	2	4	8	16	32	64	128	256	>256	MIC₅₀	MIC₇₅	MIC₉₀
Fosfomycin
Escherichia coli (n = 470)	1	0	0	1	15	302	52	24	10	15	29	16	1	4	2	4	64
Klebsiella pneumoniae (n = 429)	0	0	0	0	1	5	3	12	41	86	80	65	55	81	64	256	>256
Enterobacter species (n = 144)	0	0	0	0	0	12	4	5	11	39	32	10	17	14	64	128	>256
Proteus species (n = 262)	0	11	4	3	3	13	18	31	29	46	43	34	8	19	32	64	256
Citrobacter species (n = 45)	0	0	0	1	0	29	6	1	1	1	2	2	2	0	2	4	64
Nitrofurantoin
E. coli (n = 470)	0	0	0	1	0	0	5	44	145	178	53	29	13	2	32	32	64
K. pneumoniae (n = 429)	0	0	0	0	0	0	0	2	11	36	102	99	168	11	128	256	256
Enterobacter species (n = 144)	0	0	0	0	0	0	0	0	11	8	38	50	36	1	128	256	256
Proteus species (n = 262)	1	13	0	0	0	0	0	1	1	4	14	114	110	4	128	256	>256
Citrobacter species (n = 45)	0	0	0	0	0	0	1	0	12	16	9	6	1	0	32	64	128

MIC₅₀, MIC₇₅, MIC₉₀: concentration inhibiting 50%, 75%, and 90% of the isolates, respectively; MICs are reported in μg/mL.

MIC, minimum inhibitory concentration.

Table 2.

In Vitro Antibacterial Activities of Fosfomycin, Nitrofurantoin, and Other Antibiotic Against All Enterobacterales

Pathogen	Antibacterial agent	MIC range (μg/mL)	MIC₅₀ (μg/mL)	MIC₇₅ (μg/mL)	MIC₉₀ (μg/mL)	Susceptibility (CLSI)
Pathogen	Antibacterial agent	MIC range (μg/mL)	MIC₅₀ (μg/mL)	MIC₇₅ (μg/mL)	MIC₉₀ (μg/mL)	% S	% I	% R
Escherichia coli (n = 470)	Fosfomycin	0.06 to >256	2	4	64	95.5	3.4	1.1
	Nitrofurantoin	4 to >256	32	32	64	79.4	11.3	9.4
	Piperacillin/tazobactam	0.06 to >256	8	256	>256	56.3	9.7	34.0
	Ceftazidime/avibactam	0.06 to >256	0.12	2	128	79.8	—	20.2
	Ciprofloxacin	0.06 to 64	64	64	64	8.0	5.1	86.9
	SXT	0.03 to >256	64	256	256	31.6	—	68.4
	Ceftriaxone	0.015 to >256	256	>256	>256	17.3	0.2	82.5
	Ceftazidime	0.03 to >256	32	>256	>256	24.5	8.6	66.9
	Amikacin	1 to >256	8	32	>256	72.3	4.7	23.0
	Meropenem	0.015 to >256	0.03	0.25	64	77.6	0.2	22.2
Klebsiella pneumoniae (n = 429)	Fosfomycin	1 to >256	64	256	>256	53.1	15.2	31.7
	Nitrofurantoin	8 to >256	128	256	256	11.4	23.8	64.8
	Piperacillin/tazobactam	0.25 to >256	>256	>256	>256	36.5	2.7	60.8
	Ceftazidime/avibactam	0.06 to >256	0.5	128	128	64.2	—	35.8
	Levofloxacin	0.06 to 64	16	64	64	14.9	15.4	69.7
	SXT	0.03 to >256	128	256	256	29.0	—	71.0
	Ceftriaxone	0.03 to >256	>256	>256	>256	11.8	0.7	87.5
	Ceftazidime	2 to >256	128	>256	>256	14.0	4.0	82.0
	Amikacin	0.5 to >256	64	>256	>256	44.3	5.1	50.6
	Meropenem	0.015 to >256	16	32	128	43.9	2.7	53.5
Enterobacter spp. (n = 144)	Fosfomycin	2 to >256	64	128	>256	71.5	6.9	21.5
	Nitrofurantoin	16 to >256	128	256	256	13.2	26.4	60.4
	Piperacillin/tazobactam	0.5 to >256	8	>256	>256	55.0	2.7	42.3
	Ceftazidime/avibactam	0.06 to >256	0.5	128	128	60.3	—	39.7
	Ciprofloxacin	0.06 to 64	2	32	64	35.6	3.4	61.1
	SXT	0.06 to >256	8	256	>256	47.2	—	52.8
	Ceftazidime	0.06 to >256	32	>256	>256	39.6	3.4	57.0
	Amikacin	1 to 256	4	256	256	65.1	4.0	30.9
	Meropenem	0.015 to 256	0.06	16	64	58.4	5.4	36.2
Proteus spp. (n = 262)	Fosfomycin	0.12 to >256	32	64	256	76.7	13.0	10.3
	Nitrofurantoin	0.06 to >256	128	256	>256	7.6	5.3	87.0
	Piperacillin/tazobactam	0.06 to >256	0.25	0.5	128	85.5	4.3	10.1
	Ceftazidime/avibactam	0.12 to >256	0.12	1	256	82.6	—	17.4
	Ciprofloxacin	0.06 to 256	4	16	64	23.7	5.0	71.4
	SXT	1 to >256	256	>256	>256	31.9	—	68.1
	Ceftazidime	0.12 to >256	0.12	>256	>256	59.4	0.7	39.9
	Amikacin	0.06 to >256	8	64	>256	61.5	10.9	27.6
	Meropenem	0.03 to >256	0.06	0.06	16	85.5	0.4	14.1
Citrobacter spp. (n = 45)	Fosfomycin	0.5 to 256	2	4	64	91.1	4.4	4.4
	Nitrofurantoin	8 to 256	32	64	128	64.4	20.0	15.6
	Piperacillin/tazobactam	1 to >256	4	16	256	78.7	8.5	12.8
	Ceftazidime/avibactam	0.06 to >256	0.25	0.25	128	85.1	—	14.9
	Ciprofloxacin	0.06 to 64	2	4	64	42.6	2.1	55.3
	SXT	0.12 to 256	2	128	256	51.1	—	48.9
	Ceftriaxone	0.03 to >256	32	256	>256	40.9	2.3	56.8
	Ceftazidime	0.12 to >256	16	64	>256	44.7	2.1	53.2
	Amikacin	1 to >256	2	16	>256	74.5	2.1	23.4
	Meropenem	0.015 to >256	0.03	0.06	4	89.4	0.0	10.6

MIC₅₀, MIC₇₅, MIC₉₀: concentration inhibiting 50%, 75%, and 90% of the isolates, respectively; MICs are reported in μg/mL.

CLSI, Clinical and Laboratory Standards Institute; SXT, trimethoprim–sulfamethoxazole.

The susceptibility of E. coli expressing various β-lactamases to fosfomycin and other antibiotics was also determined. E. coli with no β-lactamase expression (CAZ sensitive) and those expressing ESBL, ESBL and class C, as well as MBL, showed high susceptibility to fosfomycin (97.4%, 94.4%, 95.7%, and 94.4%, respectively). However, E. coli strains expressing OXA-48/181 showed relatively lower activity (76.9% susceptibility). Against quinolone (ciprofloxacin MIC₅₀: 64 mg/L) and SXT-resistant E. coli (SXT MIC₅₀: >128 mg/L), fosfomycin showed MIC₅₀ of 2 mg/L and MIC₉₀ of ≤64 mg/L, which are lower than its susceptibility breakpoint (Supplementary Table S1). Unlike several other comparator agents such as PIP-TAZ, CAZ-AVI, SXT, and ciprofloxacin, fosfomycin uniquely demonstrated higher activity against E. coli strains coexpressing ESBL and class C, as well as strains expressing carbapenemase (MBLs) (Supplementary Table S1). Similarly, the susceptibility rates of K. pneumoniae, Enterobacter spp., Proteus spp., and Citrobacter spp., expressing various β-lactamases to fosfomycin were also analyzed.

The susceptibility rates of K. pneumoniae without ESBL were 77.6%, while for strains expressing ESBLs, class C+ESBL, MBL, and OXA-48/181 were 70.1, 58.8, 46.9, and 24.6, respectively (Supplementary Table S2). Diverse β-lactamase producing Enterobacter spp., Proteus spp., and Citrobacter spp., also showed good susceptibility toward fosfomycin (Supplementary Tables S3, S4, and S5). Thus, across Enterobacterales pathogens, including those expressing widely prevalent diverse β-lactamases (except for K. pneumoniae producing class C and MBL, or OXA-48/181like), showed >70% susceptibility to fosfomycin. In contrast, the susceptibility rates for carbapenemase-producing Enterobacterales were significantly lower for PIP-TAZ, ciprofloxacin, SXT, and MEM compared to fosfomycin.

The in vitro activity of nitrofurantoin was also evaluated. The high MIC₅₀/MIC₉₀ values for nitrofurantoin against E. coli, K. pneumoniae, Enterobacter spp., Proteus spp., and Citrobacter spp., suggest its limited activity (Table 1). Even though MIC₉₀ was quite high for most Enterobacterales, based on the CLSI breakpoint, 79.4% of E. coli were susceptible to nitrofurantoin. For other Enterobacterales, relatively lower susceptibility to nitrofurantoin was observed (K. pneumoniae—11.4%, Enterobacter spp.,—13.2%, Proteus spp.,—7.6%, and Citrobacter spp.,—64.4%). Further analyses of nitrofurantoin activity pertaining to various β-lactamases expressed indicated a higher susceptibility for CAZ-S, ESBL, and class C producing E. coli (83–90%); however, unlike fosfomycin, the susceptibility to nitrofurantoin for MBL expressing strains was quite limited (55%).

Discussion

In the absence of novel oral multidrug resistant-active therapy, clinicians are compelled to explore the utility of older antibiotics such as fosfomycin and nitrofurantoin for infections caused by pathogens beyond their approved pathogen spectrum.^9,15 The present study was undertaken to assess the activity of fosfomycin and nitrofurantoin against Enterobacterales to determine the pathogen group-wise susceptibility to help explore their empiric use potential for infections caused by pathogens other than E. coli. In this context, the current study examined the potency and spectrum of these agents using Enterobacterales isolates collected from several Indian tertiary care hospitals.

Applying E. coli breakpoints, except for K. pneumoniae (53.1%), the susceptibility rates for E. coli (95.5%), Enterobacter spp. (71.5%), Proteus spp. (76.7%), and Citrobacter spp. (91.1%) to fosfomycin were quite encouraging. In light of multidrug resistant nature of these pathogens and reduced susceptibility toward range of other antibiotic studied, a higher susceptibility to fosfomycin points toward its potential clinical utility (Table 2). Evaluation of fosfomycin activity against U.S. Gram-negative isolates (n = 1,400 strains) collected as a part of the SENTRY Antimicrobial Surveillance Program showed ≥90% susceptibility among K. pneumoniae, Enterobacter spp., Proteus spp., and Citrobacter spp.¹⁶ Relatively higher susceptibility rates observed in this reported study are attributable to the overall lower degree of multidrug resistance prevalent in U.S. isolates compared to Indian clinical isolates. Similar to our study, in a Canadian study by Karlowsky et al. using clinical isolates collected during 2010–2013, a susceptibility of >94% for fosfomycin was observed for urinary E. coli coproducing ESBL and class C β-lactamases.¹⁷ Number of other such studies have also demonstrated high degree of activity of fosfomycin against ESBL producing E. coli, Klebsiella, and Enterobacter.^18–23 For instance, high fosfomycin susceptibilities among Carbapenem-Resistant Enterobacterales (CREs) (except K. pneumoniae) have also been reported by Banerjee et al.²⁴

In a study by Shakti and Veeraraghavan, the resistance rates for E. coli toward nitrofurantoin have been reported to be in the range of 5% to 24.4%,²⁵ which are consistent with resistance rates observed in our study (9.4%). However, the nitrofurantoin resistance rates reported by Munoz-Davila (1.1–1.8%) are much lower than those observed in our study, which could be a reflection of overall lower resistance rates encountered in the United States, Canada, and France, the countries from which the strains were included in the Munoz-Davila study.²⁶ Unlike E. coli, the resistance rates for nitrofurantoin against K. pneumoniae were higher (64.8%), with poor coverage of ESBL, ESBL+AmpC, and MBL or OXA-48/181 producers (susceptibility rates: 17.6% for ESBL, 5.9% for class C, 7.2% for MBL, and 1.3% for OXA-48/181 like). Similar to other studies, our study also revealed some important gaps in the spectrum of fosfomycin and nitrofurantoin as evidenced by higher resistance observed in K. pneumoniae and Enterobacter spp.

The majority of Enterobacterales evaluated in our study showed high resistance rates to CAZ, ceftriaxone, fluoroquinolones, amikacin, and MEM, which are attributable to the high prevalence of multidrug resistance in India. In this context, a > 70% susceptibility (based on E. coli breakpoint) for fosfomycin, among such resistance-enriched population, reveals expanded therapeutic avenues for fosfomycin.

FDA label of fosfomycin mandates its use for the treatment of infections specifically caused by E. coli. Since E. coli is the most common pathogen in cUTI, our study suggests that fosfomycin and nitrofurantoin would continue to retain good efficacy against E. coli, including those expressing carbapenemases. However, our study further shows that, in light of marketed antibiotics failing to demonstrate comprehensive activity, fosfomycin may also possess therapeutically relevant activity against Enterobacter spp., Proteus spp., and Citrobacter spp., including carbapenemase-producing strains. This observation points toward its potential for the treatment of community infections caused by Enterobacterales other than E. coli especially for strains expressing carbapenemase for which current treatment options are extremely limited. This would potentially expand the therapeutic scope of fosfomycin as a rescue therapy.

One of the challenges in delineating therapeutic coverage of fosfomycin for diverse pathogen/resistance mechanisms is the lack of robust PK/PD studies involving Enterobacterales other than E. coli. In this context, a recent publication assumes significance, in which in vivo efficacy of fosfomycin was demonstrated against ESBL and carbapenemase expressing E. coli, K. pneumoniae, and P. aeruginosa with fosfomycin MICs ranging from 1 to 16 mg/L using neutropenic thigh infection model. However, the evidence of fosfomycin efficacy against strains with higher MIC strains is not reported (>16 mg/L). Moreover, the strains included in the study showed limited diversity in β-lactamases (E. coli producing ESBL, K. pneumoniae producing KPC-3, and P. aeruginosa producing NDM-1).²⁷ Given good susceptibility of fosfomycin toward multiple ESBL expressing E. coli, Enterobacter spp., Proteus spp., and Citrobacter spp., observed in this study, a supportive comprehensive in vivo PK/PD data could further define the role of fosfomycin in treating infections caused by non E. coli pathogens. However, any potential therapeutic use of fosfomycin for the treatment of infections beyond approved coverage profile should be considered within the confines of precautions, warnings, and past clinical experiences associated with this drug.

For nitrofurantoin, in vivo PK/PD studies are scarcely published. In one of the studies, an improved activity of nitrofurantoin was observed when the assessment was undertaken under clinically relevant experimental conditions using urine. This suggests that nitrofurantoin may demonstrate better in vivo efficacy relative to that observed under standard in vitro conditions.²⁸ In a PK/PD study designed to generate several % T > MIC scenarios using dynamic concentrations of nitrofurantoin, it was observed that T > MIC was a better predictor of nitrofurantoin PD effect against E. coli. However, these studies did not use other genera and/or multiple resistance mechanisms. Thus, in light of the lack of comprehensive in vivo PK/PD data, it is challenging to decipher the extent of the therapeutic utility of nitrofurantoin against Enterobacterales other than E. coli.

In sum, the current study provides a comprehensive report of the activity of oral therapeutic agents fosfomycin and nitrofurantoin against a large set of clinical isolates. However, the study has some limitations. This study is based on clinical isolates collected from hospitals and therefore may not represent the resistance scenario in the community. Second, the bacterial population used in the study is biased toward highly resistant strains as the source country India is inflicted with widespread multidrug resistance. Furthermore, the strains were not subjected to genetic characterization of β-lactamase genes, although phenotypic characterization was undertaken and several relevant comparator antibiotics were studied. Nevertheless, the study results provide an important reference data for monitoring future shifts in susceptibilities of fosfomycin and nitrofurantoin. Moreover, the magnitude of in vitro activity described in this study could be integrated into future PK/PD analyses/studies to judge the potential of fosfomycin in the treatment of community Gram-negative infections caused by pathogens other than E. coli.

Conclusion

Based on E. coli breakpoints, fosfomycin displayed encouraging activity against majority of clinically relevant Enterobacterales with the exception of carbapenemase expressing K. pneumoniae. In particular, fosfomycin demonstrated potent activity against E. coli strains, including those expressing carbapenemases. Notably, since the panel of Indian Enterobacterales strains used in this study was enriched for carbapenem-impacting resistance mechanisms, several marketed and newer Gram-negative antibiotics demonstrated poor activity. In this context, a > 70% susceptibility among Enterobacter spp., Proteus spp., and Citrobacter spp., including those expressing carbapenemases observed for fosfomycin, points toward its expanded therapeutic potential for pathogens other than E. coli in high resistance settings.

Additional robust PK/PD studies could complement susceptibility studies such as the present one to judge the potential for expanded therapeutic utility of fosfomycin. However, the therapeutic use of the drug for infections caused by pathogens beyond the approved label must be considered within the confines of precautions, warnings, and past clinical experiences associated with this drug. In contrast, nitrofurantoin showed a relatively narrow spectrum of activity limited to ESBL/class C β-lactamases expressing E. coli with carbapenemase expressing E. coli being out of bound for this drug.

Footnotes

Acknowledgments

The authors thank the Wockhardt Research Centre, India, and Dr. Mahesh Patel for granting permission to conduct these studies. The authors also acknowledge the excellent technical assistance from Mr. Vineet Zope, Mr. Prashant Joshi, Mr. Melroy Tellis, and Mr. Devendra Kulkarni.

Disclosure Statement

No competing financial interests exist.

Funding Information

This work was self-funded.

Supplementary Material

References

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