Short Communication
Differences in tetracycline resistance determinant carriage among Shigella flexneri and Shigella sonnei are not related to different plasmid Inc-type carriage

https://doi.org/10.1016/j.jgar.2017.12.015Get rights and content

Highlights

  • tet(B) was the most frequently detected tetracycline resistance determinant in Shigella isolates.

  • The tet(A) gene was more frequent in Shigella sonnei.

  • The tet(B) gene was more frequent in Shigella flexneri.

  • Plasmid incompatibility (Inc) groups showed a species-specific distribution.

  • No relationship between plasmid Inc groups and tet genes was found.

Abstract

Objectives

The aim of this study was to establish the prevalence of the most common molecular mechanisms involved in tetracycline resistance as well as their relationship with plasmid incompatibility (Inc) groups in a collection of Shigella spp. causing traveller’s diarrhoea.

Methods

Tetracycline susceptibility was established in 187 Shigella spp. (74 Shigella flexneri and 113 Shigella sonnei), of which 153 isolates were recovered as a confirmed cause of traveller’s diarrhoea. The prevalence of the tet(A), tet(B) and tet(G) genes was analysed by PCR. Eighteen plasmid Inc groups was determined in a subset of 59 isolates.

Results

Among 154 tetracycline-resistant isolates, 122 (79.2%) harboured at least tet(A) or tet(B). The tet(B) gene was the most frequently detected, being present in 70 isolates (45.5%), whilst tet(A) was detected in 57 isolates (37.0%). The tet(G) gene was present in only 11 (7.2%) isolates. Moreover, the tet(A) gene was more frequent in S. sonnei (P = 0.0007), whilst the tet(B) gene was more frequent in S. flexneri (P < 0.0001). Plasmids belonging to Inc group B (P < 0.05) were significantly more frequent among S. flexneri, whilst those belonging to groups K, FIC and FIIA (P < 0.05) were preferentially detected among S. sonnei.

Conclusion

The prevalence of the tet(A) and tet(B) genes differed between S. sonnei and S. flexneri. Moreover, the prevalence of plasmid Inc groups in S. flexneri and S. sonnei differed. However, no relationship was found between the two phenomena.

Introduction

Antimicrobial resistance is continuously increasing worldwide. Mechanisms of antimicrobial resistance may be found in pristine environments such as artic soils [1]. None the less, there is a direct relationship between local human and veterinary antibiotic use patterns and specific local bacterial antimicrobial resistance profiles. However, international travel and commerce facilitate the distant spread of pathogenic, commensal and environmental micro-organisms that may carry antibiotic resistance determinants [2], [3], [4], [5]. This dissemination of bacteria/genes in a specific geographical area may result in the detection of micro-organisms exhibiting resistance to antimicrobial agents that are not usual to that area, such as what has been described in remote Peruvian rural regions [6].

In this context, international travellers carry micro-organisms from their geographical area to that of their travel destination, and vice versa. However, this international dissemination of micro-organisms usually remains undetected since commensals do not produce disease, and this dissemination only comes to attention when a pathogenic micro-organism is acquired [5].

Traveller’s diarrhoea (TD) is the most frequent illness affecting travellers to developing areas. A series of pathogens have been described underlying this illness, although bacteria are by far the most relevant aetiological cause [7]. Diarrheagenic Escherichia coli and Shigella spp. rank among the most frequent micro-organisms described in TD patients and are also of special relevance because antibacterial agents are often used due to the symptom duration or the severity of illness [8]. However, differences in the geographical prevalence of Shigella flexneri and Shigella sonnei have also been described [9], [10].

Recent studies demonstrate an overall increase in the antibiotic resistance levels of Shigella spp. causing TD, with tetracycline resistance levels increasing from 77.1% in the period 1995–2000 to 92.5% in the period 2001–2010 [10]. Tetracycline resistance is mediated by different mechanisms (efflux pumps, ribosomal protection, enzyme inactivation, impermeability or target alterations), most of which are encoded within transferable elements, with wide dissemination within different bacteria [11].

Previous studies have shown differences in the antibiotic resistance levels of S. flexneri and S. sonnei [10], [12] as well as in the prevalence of transferable antibiotic resistance determinants. Differential species-specific blaOXA genes have been described among S. flexneri and S. sonnei causing either TD or infections in specific areas [3], [12]. Regarding tetracycline, the most relevant mechanisms of resistance in Shigella spp. are those related to transferable efflux pumps [11], with the tet(A) and tet(B) genes identified in 80–100% of analysed tetracycline-resistant Shigella isolates [12], [13], [14], [15]. Other resistance determinants, such as tet(C) or tet(D), are rarely detected alone [12], [13], [14]. Finally, determinants such as tet(G) have also been detected [12], [13], [14] but remain understudied. Although local studies have shown differences between the tetracycline resistance determinants present in S. flexneri and S. sonnei [12], [15], there are no data on isolates causing TD, which reflect a scenario more pertinent to human health.

Antibiotic resistance determinants, such as those involved in tetracycline resistance, are often encoded within transferable elements such as plasmids. Plasmids are classified within the so-called ‘incompatibility (Inc) groups’ [16]. Within each group, plasmids are mutually exclusive. Interestingly, several antibiotic resistance determinants (blaCTX-M-2 and blaCTX-M-19) have been associated with different plasmid Inc groups [17]. This association might be extended to other antibiotic resistance determinants, such as the tet determinants, and thus explain differences in tet determinant carriage among different Shigella spp., resulting in a possible barrier to horizontal transfer of tet(A) or tet(B) between Shigella spp.

Therefore, a surveillance study of different mechanisms of tetracycline resistance was performed in a subset of Shigella spp. associated with TD in order to investigate differences in Shigella spp. tetracycline resistance gene carriage, which may be related to geographical origin and/or the presence of barriers to horizontal gene transfer between Shigella spp.

Thus, the aim of this study was to establish the prevalence of tet(A), tet(B) and tet(G) genes in a collection of Shigella spp. recovered from 1995–2010 as a cause of TD and to determine their association with plasmid Inc groups.

Section snippets

Strains

A total of 187 Shigella spp. isolates (74 S. flexneri and 113 S. sonnei) recovered between 1995 and 2010 at the Microbiology Service of the Hospital Clinic of Barcelona (Barcelona, Spain) were included in this study, of which 153 isolates (60 S. flexneri and 93 S. sonnei) were isolated from patients with TD visiting the Tropical Medicine Unit. Travel history was collected from an internal database in all cases except for 34 (14 S. flexneri and 20 S. sonnei).

Antimicrobial susceptibility testing

In these travel-related isolates,

Results and discussion

Currently at least 59 tetracycline resistance determinants have been described, with 28 being present in Gram-negative micro-organisms [11]. Studies determining the presence of these genes among Enterobacteriaceae have described a great variety of genes, particularly the tet(A) and tet(B) genes [11], [12]. This is also confirmed in the present study, with 154 tetracycline-resistant isolates (62 S. flexneri and 92 S. sonnei) plus 1 tetracycline-intermediate S. sonnei isolate being identified. Of

Conclusions

In summary, the present study reveals that while the tet(A) and tet(B) genes were frequently detected as a cause of tetracycline resistance in Shigella spp. causing TD, the presence of tet(G) gene was infrequent. Although differences in the Inc plasmid groups carried by S. sonnei and S. flexneri were observed and the prevalence of the tet(A) and tet(B) genes differed between S. sonnei and S. flexneri, no specific associations between tet determinants and the Inc plasmid groups were observed.

Acknowledgment

The authors thank Donna Pringle for editorial assistance.

References (20)

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Cited by (5)

1

Present address: Department of Medical Microbiology, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.

2

Present address: Molecular Microbiology Area, CIBIR, Logroño, Spain.

3

Present address: Laboratory of Virus Contaminants of Water and Food, Department of Microbiology, University of Barcelona, Barcelona, Catalonia, Spain.

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