Assessment of Antibiotic Resistance in Probiotic Lactobacilli

Probiotics are live microorganisms that confer a health benefit on the host when administered in adequate amounts (Food and Agriculture Organization of the United Nations-World Health Organization Working Group, 2002). Many microbial species have probiotic properties, but those most commonly used are lactobacilli (Salminen et al, 1998; Caplice & Fitzgerald, 1999; Leroy & De Vuyst, 2004). Lactobacilli have a long history of safe use in the production and consumption of fermented foods and beverages. Over recent decades, as awareness of the beneficial effects of probiotic strains in promoting gut and general health has grown, the development and consumption of probiotic foods has increased worldwide (Saarela et al, 2002). Thus, it is essential to thoroughly investigate the safety of lactobacilli used in probiotic products (Salminen et al, 1998; Borriello et al, 2003).


Introduction
Probiotics are live microorganisms that confer a health benefit on the host when administered in adequate amounts (Food and Agriculture Organization of the United Nations-World Health Organization Working Group, 2002).Many microbial species have probiotic properties, but those most commonly used are lactobacilli (Salminen et al, 1998;Caplice & Fitzgerald, 1999;Leroy & De Vuyst, 2004).Lactobacilli have a long history of safe use in the production and consumption of fermented foods and beverages.Over recent decades, as awareness of the beneficial effects of probiotic strains in promoting gut and general health has grown, the development and consumption of probiotic foods has increased worldwide (Saarela et al, 2002).Thus, it is essential to thoroughly investigate the safety of lactobacilli used in probiotic products (Salminen et al, 1998;Borriello et al, 2003).
The human gut is the natural habitat for a large and dynamic bacterial community that has a great relevance for health (Spor et al, 2011).The human gut microbiota is a complex ecosystem colonized by approximately 10 14 bacterial cells with Bacteroides, Eubacterium, Bifidobacterium, Ruminococcus, and Clostridium as the pre-dominant genera (Kurokawa et al, 2007).The huge diversity of antibiotic resistance genes detected in the human gut microbiome suggests that antibiotic resistant bacteria in the gastrointestinal tract (GIT) function as reservoir of antibiotic resistance genes (Salyers et al, 2007;Sommer et al, 2009).When probiotic strains enter the gut, they interact with the native microbiota and gene transfer can occur (Teuber et al, 1999;Mathur & Singh, 2005;Salim Ammor et al, 2007).The dissemination of antibiotic resistance genes can reduce the therapeutic possibilities in infectious diseases.It is therefore relevant to look for the presence of transferable antibiotic resistance genes in lactobacilli that are or shall be used as probiotic strains for human consumption or as starter cultures of fermented food or feed products.This article reviews the experiments to be performed and the criteria for assessment of antibiotic resistance in probiotic lactobacilli.Due to the growing availability of whole bacterial genome sequences, sequence-based identification approaches for antibiotic resistance are also discussed.

Antibiotic resistance of probiotic lactobacilli
Many food production is estimated to involve microbial fermentation processes by using lactic acid bacterial (LAB) strains (Food and Agriculture Organization of the United

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Nations-World Health Organization Working Group, 2002), for example, sausage, ham, cheese, and dairy products.In addition, probiotics have become available on the market, containing a single strain or a combination of strains.The proposed problem is that probiotic strains might contain acquired resistance genes.From a point of safety, it is necessary to distinguish between intrinsic and acquired resistance genes.
Antibiotic resistance profiles have recently been reported for several lactobailli.These have been found susceptible to penicillins and ampicillin (cell wall synthesis inhibitor) (Danielsen & Wind, 2003;Coppola et al, 2005) in contrast to vancomycin.Most lactobacilli have been found to be resistant to glycopeptides types of antibiotics.However, the resistance towards vancomycin has been demonstrated being as intrinsic (Tynkkynen et al, 1998).Lactobacilli are usually susceptible to chloramphenicol, erythromycin and clindamycin (protein synthesis inhibitors) (Coppola et al, 2005;Klare et al, 2007).In addition, resistance against trimethoprim (nucleic acid synthesis inhibitor), seems to be intrinsic (Ammor et al, 2007).Resistance to tetracycline has been observed more often among lactobacilli (Roberts, 2005;Korhonen et al, 2008).Resistance against neomycin, kanamycin, streptomycin and gentamicin (aminoglycosides) has been observed more frequently among lactobacilli (Coppola et al, 2005;Danielsen, 2002;Zhou et al, 2005).
Acquired resistance genes which are potentially transferable have been detected in lactobailli.These have been described in multiple studies and have been reviewed (Ammor et al, 2007).Two of the most commonly observed resistance genes in lactobacilli found so far are tet(M) for tetracycline resistance and erm(B) for erythromycin resistance, followed with cat genes coding for chloramphenicol resistance (Danielsen, 2002;Lin et al, 1996;Gevers et al, 2003a;Cataloluk & Gogebakan, 2004).
Acquired resistance genes of probiotic lactobacilli have been reported previously (Table 1).In the PROSAFE project, probiotic lactobacilli possessed erm(B) and/or tet(W), tet(M) or unidentified members of the tet(M) group (Klare et al, 2007).In probiotic commercial L. reuteri ATCC 55730, tet(W) and the lincosamide resistance gene lnu(A) were detected (Kastner et al, 2006).Hummel et al determined antibiotic resistances of probiotic lactobacilli and to verify these at the genetic level.L. salivarius BFE 7441 possessed an erm(B) gene, which was encoded on the chromosome (Hummel et al, 2007).Probiotic lactobacilli of African and European origins were studied and compared for their susceptibility to antibiotics.Acquired resistance genes encoding aminoglycoside (aph(3')-III, aadA, aadE) and tet(S) and erm(B) were detected (Ouoba et al, 2008).The potentially probiotic strain L. plantarum CCUG 43738, which displayed atypical phenotypic resistance to tetracycline and minocycline, was found to contain a tet(S) gene located on a plasmid of approximately 14 kb (Huys et al, 2006).

Evidence of potential horizontal gene transfer of probiotic lactobacilli
When probiotic strains enter the gut, they interact with the native microbiota and gene transfer can occur.Probiotics might contribute to the transfer of antibiotic resistance genes to other commensal bacteria or pathogens present in the GIT.The occurrence of large numbers of transferable resistance genes within the intestinal microbiota is undesirable due to the potential risk of acquisition by pathogens present in the GIT and subsequent antibiotic treatment failure (Licht & Wilcks, 2005) No transconjugants in mating experiment Ouoba et al, 2008 L. plantarum L7 Am r , Ci r , Gm r , Tc r , Km r , Sm r , Vm r aadE No transconjugants in mating experiment Ouoba et al, 2008 L. casei L9 Am r , Ci r , Gm r , Km r , Sm r , Vm r
Transfer has been demonstrated in the GIT of rodents, both gnotobiotic (Feld et al, 2008;Jacobsen et al, 2007;Morelli et al, 1988) and those having an indigenous gut microbiota (Feld et al, 2008;Jacobsen et al, 2007;Schlundt et al, 1994;McConnell et al, 1991;Gruzza et al, 1994;Igimi et al, 1996).In addition, the in vivo transfer of vancomycin resistance has recently been shown between enterococci and probiotic lactobacilli in gnotobiotic mice (Mater et al, 2008).
Recent experiments of antibiotic resistance transferability in vivo were also conducted from L. plantarum to E. faecalis (Jacobsen et al, 2007).However, the potential contribution of lactobacilli to the acquisition and dissemination of antibiotic resistance genes in the human GIT is poorly addressed for both conjugative and non-conjugative resistance plasmids.Nevertheless, conclusive documentation of transfer in the GI from probiotic lactobacilli is lacking and therefore more studies need to be carried out.

Systematic assessment of antibiotic resistance in probiotic lactobacilli
Antibiotic-resistance screening for lactobacilli intended for use in dairy products such as probiotics or as starters is now tending to become systematic.The European Food Safety Authority (EFSA) has taken responsibility to launch the European initiative toward a "qualified presumption of safety" (QPS) concept which, similar to the GRAS system in the United States, is aimed to allow strains with an established history and safety status to enter the market without extensive testing requirements (European Commission, 2003).The QPS approach together with the recommendations of the FEEDAP panel of EFSA will give a framework for better decision making in safety assessments of antibiotic resistance (Figure 1) (European Commission, 2005;European Food Safety Authority, 2008).
In phenotypic methods, FEEDAP requires the determination of the MICs of the most relevant antibiotics for each bacterial strain that is used as a feed additive in order to eliminate the possibility of acquired resistances.Those microbiological breakpoints define a MICs which, if exceeded, triggers the need for a more extensive investigation to define the genetic basis of the observed resistance and to assess the risk for transfer of this resistance to other bacteria.In genotypic methods, the latest literature indicates that the search for acquired resistance genes using PCR-based techniques (Klare et al, 2007;Hummel et al, 2007;Ouoba et al, 2008;Ammor et al, 2008;Devirgiliis et al, 2008;Fukao et al, 2009;Rizzotti et al, 2009;Comunian et al, 2010) or micro-arrays (Ammor et al, 2008) is a powerful tool to identify resistant LAB strains.In case of suspected acquired resistance or intrinsic resistance, transferability tests are optional.Conjugation can be detected with bacterial mating experiments.The suspected donor with an antibiotic resistant phenotype is mixed with a recipient strain sensitive to the respective antibiotic, and the transfer of the resistance is subsequently checked.Frequencies of 10 -6 to 10 -5 of transconjugant cells are usually the highest experimentally obtainable.If transferability of the resistance is proven, then the strain will not be considered for use in microbial products and further tests are superfluous.
Systematic assessment of antibiotic resistance in some commercial probiotic lactobacilli have been reported previously (Table 1).L. rhamnosus GG (ATCC 53103) is a probiotic strain used in fermented dairy products in many countries.Studies have shown that the genes needed for vancomycin resistance in L. rhamnosus GG are not related to transferable enterococcal van genes and have not revealed any potential risks caused by the vancomycin resistance in this strain (Tynkkynen et al, 1998).The QPS approach was applied to determine the resistance of the probiotic strain Lactobacillus brevis KB290 that is used as a probiotic strain in fermented food products in Japan.The authors concluded from their investigation that the antibiotic resistance observed in L. brevis KB290 was due not to a potentially acquired mechanisms but to intrinsic resistance.It was concluded that according to the QPS criteria, these results provided safety assurance for the ongoing use of L. brevis KB290 as a probiotic (Fukao et al, 2009).In the PROSAFE project, probiotic lactobacilli displayed phenotypic resistance to tetracycline and/or erythromycin possessed erm(B) and/or tet(W), tet(M) or unidentified members of the tet(M) group.In vitro intra-and interspecies filter-mating experiments failed to show transfer of resistance determinants.L. reuteri ATCC 55730, a commercially available, well-documented a probiotic bacterium, has been shown to carry unusual resistances to tetracycline and lincosamides (Kastner et al, 2006).Deletion of the two plasmids was achieved by use of a protoplast-formation technique.BioGaia concluded that L. reuteri strain DSM 17938, except for the deletion of plasmids pLR581 and pLR585, was substantially equivalent to its parent strain L. reuteri ATCC 55730.Additionally, BioGaia concluded that the evidence demonstrating the safety of strain ATCC 55730 is equally applicable to strain DSM 17938 (Rosander et al, 2008).Hummel et al determined antibiotic resistances and to verify these at the genetic level according to the QPS system.L. salivarius BFE 7441 possessed an erm(B) gene, which was encoded on the chromosome and which could not be transferred in filter-mating experiments (Hummel et al, 2007).Probiotic lactobacilli of African and European origins were studied and compared for their susceptibility to antibiotics.Acquired antibiotic resistance genes encoding aminoglycoside (aph(3')-III, aadA, aadE) and tet(S) and erm(B) were detected.Only the erm(B) gene found in L. reuteri 12002 could be transferred in vitro to enterococci (Ouoba et al, 2008).

Whole genome based assessment of antibiotic resistance in probiotic lactobacilli
Due to the growing availability of whole bacterial genome sequences, sequence-based identification approaches have in recent years been intensively explored for safety evaluation such as antibiotic resistance in probiotic lactobacilli.Commercial probiotic L. acidophilus NCFM and L. reuteri DSM 17938 were assessed with whole genome and no known acquired resistance genes were detected (Agency Response Letter GRAS Notice No. GRN 000357; (Heimbach, 2008).Whole genome sequences were used to screen for acquired antibiotic resistance genes in lactobacilli strains which could be used in human nutrition (Bennedsen et al, 2011).
Moreover the overall NCBI clusters of orthologous groups (COGs) analysis is recommended (Heimbach, 2008).The COG category V (termed defense mechanisms) consists of many COGs that may have a potential safety interest, such as antibiotic resistance (Heimbach, 2008).Although it doesn't imply that these genes in COG category V are involved in antibiotic resistance, it is recommended to be assessed that there is nothing unusual about the number of COGs belonging to category V and none of the each gene was a part of a detectable mobile element such as predicted transposase genes (Heimbach, 2008).The overall COG analysis of L. reuteri DSM 17938 with complete genomes revealed that several COGs belonging to category V were found (Heimbach, 2008).The data indicated that there was nothing unusual about the number of COGs belonging to category V among these strains.Further analysis of each of the genes revealed that no gene was clustered with complete transposons or ISs.Thus none of the genes was a part of a detectable mobile element (Heimbach, 2008).

Conclusions
In this context, probiotic lactobacilli are considered to pool the resistant genes and might transfer these to pathogenic bacteria.In order to eliminate this possibility, resistance to the most relevant antibiotics for each strain used as probiotic lactobacilli, food or feed additives could be determined using the systematic QPS protocols.Moreover, due to the growing availability of whole bacterial genome sequences, sequence-based identification approaches have been enployed.These can be used to screen strains for unwanted genetic content such as antibiotic resistance.This screening supports normal safety assessment of probiotic lactobacilli.

Fig. 1 .
Fig. 1.Proposed scheme for the antibiotic resistance assessment of a bacterial strain (European Commission, 2005; European Food Safety Authority, 2008) Antibiotic Resistant Bacteria -A Continuous Challenge in the New Millennium www.intechopen.com .