High risk of intestinal colonization with ESBL-producing Escherichia coli among soldiers of military contingents in specific geographic regions

One-hundred Polish soldiers of a contingent in Afghanistan in 2019 were screened for Enterobacterales resistant to newer-generation β-lactams at their departure and return. Seventeen percent were colonized in the gut at the departure, whereas 70% acquired carriage in Afghanistan. The commonest organisms were extended-spectrum β-lactamase (ESBL)-producing Escherichia coli (ESBL-Ec; 96.6%). All isolates were sequenced and were clonally diverse overall, even within the same sequence type, indicating that independent acquisitions mainly. ESBL-Ec were often multi-drug-resistant. Soldiers stationing in certain regions are at high risk of acquiring resistant bacteria that may cause endogenous infection, be transmitted to vulnerable individuals, and spread resistance genes.


Introduction
Polish soldiers have been participating in international missions in different countries, often in world regions of broad dissemination of antimicrobial-resistant (AMR) pathogens, such as extended-spectrum β-lactamase (ESBL)-producing Enterobacterales [1][2][3][4].Staying in such areas has been shown to be of increased risk of gut colonization with these.For example, around 75%, 49%, and 43% of Dutch travelers to Southern, Central-Eastern, and Western Asia, respectively, acquired such organisms, most often ESBLproducing Escherichia coli (ESBL-Ec) with the CTX-M-15 enzyme [2].The percentage of ESBL (CTX-M-15)-Ec carriers among French soldiers stationing in Afghanistan for one year was 34.5% [5]; however, research on AMR pathogens in military contingents has been scarce so far.Various factors may specifically facilitate acquisition of pathogens in such populations, including gathering for prolonged periods, shared social areas of limited size, common sources of food and water, and/or lower sanitary standards in the field and combat conditions.
The E. coli intestinal carriage is a significant source of endogenous infection [3,[6][7][8][9][10], and diseases caused by ESBL-Ec strains are difficult to treat because of their usual multi-drug-resistance (MDR) [3,4,9,[11][12][13].The global increase in the ESBL-Ec occurrence, raising serious clinical and epidemiological concern, has been largely due to horizontal ESBL gene transmission and/or clonal spread [3,9,12], the latter often associated with specific MDR clones, like E. coli ST131 [14].ESBLs have been the main factor of enterobacterial resistance to expanded-spectrum β-lactams (cephalosporins), followed by acquired AmpC-like cephalosporinases and, recently, carbapenemases [15].Our aim was to investigate the intestinal carrier status and characteristics of Enterobacterales resistant to newer β-lactams isolated from Polish soldiers of a contingent in Afghanistan.

Materials and methods
From June to November 2019, 295 Polish soldiers served in Afghanistan, 100 of whom, without any symptoms of illness, were tested both before and after the 6-month stay.Their stool samples were inoculated on chromID™ ESBL, CHROMID® Carba and CHROMID® OXA-48 media (bioMérieux, Marcy l'Etoile, France).Subsequently, bacterial isolates (1-4 per sample) were screened for ESBL and AmpC production, using the ESBL double-disk synergy test (including cefepime disk) [16], followed by PCRs for the detection of various ESBL or AmpC genes [17].Carbapenemase activity was assessed by the CIM assay [18].Species were identified using Vitek 2 (bioMérieux).
The resistome analysis of the ESBL-Ec strains revealed various mobile AMR genes (Table 4), which with specific chromosomal mutations corresponded well to the susceptibility patterns.Along with genes of β-lactamases mentioned above, multiple quinolone resistance determinants were common, including combinations of mutations in the chromosomal gyrA and/or parC/E genes, and acquired qnrB/S-like genes.Doxycycline, trimethoprim, and sulfamethoxazole resistance correlated with tet(A), dfr-, and sul-like genes, respectively.The isolates carried various genes of aminoglycoside-modifying enzymes; however, those conferring resistance to clinically-relevant drugs, namely, amikacin, gentamicin, and tobramycin (e.g., aac(3)-IId; aac(6')-Ib-cr), were rare.

Discussion
Our analysis revealed the 70% rate of acquisition of ESBL/AmpC Enterobacterales by a group of Polish soldiers stationing in Afghanistan in 2019.Despite some methodological differences between the studies, this high percentage doubled the 34.5% reported for French soldiers in Afghanistan in 2011 [5], but was similar to the 75% rate of ESBL-Ec acquisition by Dutch citizens travelling in 2012-2013 to the South Asia region, comprising the Afghanistan territory [2].The notable 17% carriage rate at the departure is difficult to comment because the data on the ESBL-Ec colonization in Polish community has been scarce.A study performed in 2015-2017 in several North European countries included 86 individuals (e.g., primary care patients) from Poland, 8% of whom were colonized [24], comparably to Swedish citizens (6.6%) in the same analysis, or to German population (6.3%) in another work [25].The notably higher rate in the soldiers might have been due to their clustering within units, including increased risk of common exposure to contaminated food or environment.Only 3/17 "pre-colonized" soldiers came back with the same ESBL-Ec from Afghanistan, nine had other organisms, and five were negative, which corresponded well to recent reports on high dynamics of the ESBL-Ec carriage in travelers to the "ESBL broad-dissemination" regions [26,27].Moreover, the only three cases of longer-term ESBL-Ec persistence were concordant with the data on rather short duration of the travel-acquired colonization, with a median of 1-3 months [26,28,29].The 90 ESBL/AmpC organisms from 78 individuals were sequenced.The predominance of ESBL-Ec (~ 96.6%), and the high prevalence of the CTX-M-15 enzyme (~ 94.3%) were congruent with multiple data sets on the general community and, especially, travelers to the "ESBL broad-dissemination" areas [2,5,24,25,27,[29][30][31].Our study showed several cases of the occurrence of the same organism in different soldiers, including the cluster of six E. coli ST515 CTX-M-15 at the departure, and five pairs of E. coli or K. pneumoniae CTX-M-15 at return.These evidenced either transmission of bacteria between the individuals or their acquisition from a common contaminated source.However, the overall high clonal diversity of the organisms indicated their vast majority to have been acquired independently by the participants of the military contingent.
The ESBL/AmpC-Ec diversity was demonstrated by classification of the 17 departure and 70 return isolates into nine and 38 STs, respectively, and by limited or no relatedness between most of the isolates of the same ST.The more frequent STs among the return isolates included the ubiquitous commensal ST10 and common pathogenic ST69 and ST131 lineages [14,32].However, the ~11% contribution of ST131 was much lower than in community-acquired and healthcare-associated ESBL-Ec infections [33,34], as well as carriage in hospitalized patients [17].Studies on ESBL-Ec colonizing travelers to the "ESBL broad-dissemination" regions usually showed their clonal diversity and unique ST distributions, with limited ST131 incidence [27,30,35,36].
Military contingents in some regions of the world have been at increased risk of colonization with AMR microorganisms, especially ESBL-Ec.Even though all of the carriers identified in our study were healthy individuals, they might also transmit these to vulnerable subjects in their habitats after return, creating so a significant epidemiological threat.

Table 1
Frequency and types of organisms isolated from the intestinal colonization of 100 Polish soldiers of the contingent in Afghanistan, June-November 2019 a 3/100 tested soldiers had the same or almost the same CTX-M-15-producing isolates in June and November; these were not included in the final analysis for November

Table 2
Phylogroups and STs of 87 ESBL-Ec

Table 4
Acquired AMR genes and fluoroquinolone resistance mutations identified in ESBL-Ec strains isolated from soldiers in June and November