CN112961805B - Salmonella typhimurium with quinolone drug resistance genes gyrA and parE mutated simultaneously and application thereof - Google Patents
Salmonella typhimurium with quinolone drug resistance genes gyrA and parE mutated simultaneously and application thereof Download PDFInfo
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Abstract
The invention relates to the technical field of drug-resistant strains, in particular to salmonella typhimurium with quinolone drug-resistant genes gyrA and parE mutated simultaneously and application thereof. The salmonella typhimurium simultaneously carries two quinolone drug resistance mutation sites, wherein the two quinolone drug resistance mutation sites are respectively as follows: the 83 th serine of the encoded product of the gyrA gene is mutated into phenylalanine; the 333 rd threonine of the product encoded by the parE gene was mutated to isoleucine. These mutation sites cause salmonella to exhibit drug resistance or intermediate resistance to quinolone drugs. Therefore, the salmonella provided by the invention can be used as a model material for screening novel antibacterial drugs, and has a good application prospect.
Description
Technical Field
The invention relates to the technical field of drug-resistant strains, in particular to salmonella typhimurium with quinolone drug-resistant genes gyrA and parE mutated simultaneously and application thereof.
Background
Salmonella (Salmonella spp) is a zoonosis pathogenic bacterium with important public health, can cause a plurality of serious diseases such as typhoid fever, paratyphoid fever, gastroenteritis, septicemia and the like of human and animals, and causes huge economic loss in the world. Currently, about 2610 serotypes have been found worldwide. Among them, Salmonella Typhimurium (Salmonella enterica subsp. enterica serovar Typhimurium) is the predominant serotype present in livestock and food, and is also the major serotype causing human infection.
Quinolone drugs, especially fluoroquinolone drugs, are the last important class of drugs to be selected for the treatment of potentially life-threatening infections caused by multidrug-resistant salmonella. With the wide clinical and production applications of these drugs, the problem of drug resistance of bacteria to these drugs is becoming more serious. Research shows that the drug resistance of the salmonella to quinolone drugs rises year by year, and the drug resistance rate of the salmonella typhimurium is generally higher than that of other salmonella.
The main mechanism of the salmonella for generating drug resistance to the quinolone drugs is that DNA helicases (coding genes: gyrA and gyrB) and topoisomerase IV (coding genes: parC and parE) at target sites of action of the quinolone drugs generate gene mutation, so that the enzymes generate structural change, and the combination of the enzymes and the quinolone drugs is influenced. gyrA, gyrB, parC and parE are also known as quinolone resistance-determining regions (QRDR). In addition, plasmid-mediated quinolone drug resistance mechanisms (PMQR), including QNR proteins (encoding genes: qnrA, qnrB, QNR C, qnrD, qnrS, qnrVC), efflux pump transporters (encoding genes: qepA and oqxAB), and acetyl transferase (encoding genes: aac (6') -Ib-cr) are also responsible for the decrease in the sensitivity of bacteria to quinolone drugs.
The degree of resistance of salmonella to quinolone drugs is generally related to the position and number of mutation sites. Single site mutations can cause low levels of resistance, and if 2 or more mutations occur simultaneously, they can result in the production of high levels of resistant strains. At present, in the reports about salmonella QRDR mutant strains, single-site mutation is mainly used, and a plurality of mutations occur simultaneously, so that high-level drug-resistant strains are rare.
Disclosure of Invention
The salmonella provided by the invention simultaneously carries two quinolone drug resistance mutation sites, can be used as a model material for screening functional microorganisms/novel antibacterial drugs, and has a good application prospect.
In order to achieve the purpose, the invention adopts the technical scheme that: providing salmonella typhimurium, wherein the salmonella typhimurium simultaneously carries two quinolone drug resistance mutation sites, and the two quinolone drug resistance mutation sites are respectively as follows:
the 83 th serine of the encoded product of the gyrA gene is mutated into phenylalanine;
the 333 rd threonine of the product encoded by the parE gene was mutated to isoleucine.
As a preferred embodiment of the salmonella typhimurium of the present invention, the two quinolone drug resistance mutation sites are specifically:
the 324 th base C of the gyrA gene is mutated into T;
the 998 th base C of the parE gene was mutated to T.
As a preferred embodiment of the Salmonella typhimurium of the present invention, the NCBI accession number of the gyrA gene is AE006468REGION:2373710..2376422, and the NCBI accession number of the parE gene is AE006468REGION:3343969.. 3345861.
As a preferred embodiment of the Salmonella Typhimurium of the present invention, the Salmonella Typhimurium is Salmonella Typhimurium (Salmonella enterica subsp. enterica serovar Typhimurium) Sal103t, which is classified and named as Salmonella Typhimurium (Salmonella enterica subsp. enterica serovar Typhimurium), and has been deposited at the Guangdong province collection of microorganisms and strains on day 27/10 of 2020 at the deposition address: the preservation number of building 5 of Ji 59 of Ji 100 of the prefecture Zhonglu city: GDMCC No: 61247.
the invention discovers that one strain simultaneously carries two drug-resistant mutation sites gyrA of quinolone drugsS83FAnd parET333IThe Salmonella typhimurium of (1). The gyrA gene coding product of the strain generates single-site mutation: ser83 → Ph e (serine mutated to phenylalanine, abbreviated S83F), while the parE gene-encoded product undergoes a single-site mutation (Thr333 → Ile) (threonine mutated to isoleucine, abbreviated T333I). Wherein, parET333IIs the first reported new mutation site found in this study. It is the firstSecondly, finding drug-resistant mutation site gyrA of quinolone drugsS83FAnd parET333IMeanwhile, the strain exists in a salmonella typhimurium strain, and the strain is not reported at home and abroad. Meanwhile, the strain does not have any plasmid-mediated quinolone drug resistance gene. Therefore, the mutation sites cause the salmonella Sal103t strain to show drug resistance or intermediate drug resistance to quinolone drugs, and the minimum inhibitory concentrations of nalidixic acid, ciprofloxacin, levofloxacin and moxifloxacin to the strain are 4096 mug/mL (drug resistance), 1 mug/mL (intermediate drug resistance) and 1 mug/mL (intermediate drug resistance), respectively.
The salmonella Sal103t is gram-negative brevibacterium, API 20E is identified as salmonella, and the coincidence rate is 99.9%. The biochemical characteristics are as follows: arginine double hydrolase, lysine decarboxylase and ornithine decarboxylase are positive, citrate is decomposed, hydrogen sulfide is generated, glucose, mannitol, inositol, sorbitol, rhamnose, melibiose, arabinose, urease, phenylalanine deaminase, oxidase, ONPG tests, indole tests and VP tests are negative, gelatin cannot be liquefied, cane sugar and amygdalin are not fermented, and the salmonella typical physiological and biochemical characteristics are provided. The serum antigen is identified as 1,4,5,12: i:1,2, a typical serotype of Salmonella typhimurium.
The Salmonella Sal103t of the present invention can be cultured in LB, BHI and NA media.
The invention also provides a microbial agent, which comprises the salmonella typhimurium.
The invention also provides application of the salmonella typhimurium in screening a model strain of a novel antibacterial drug.
The invention has the beneficial effects that:
the salmonella Sal103t simultaneously carries two drug-resistant mutation sites gyrA of quinolone drugsS83FAnd parET333IThe Salmonella typhimurium of (1). The gyrA gene coding product of the strain generates single-site mutation: ser83 → Phe (serine mutated to phenylalanine, abbreviated S83F), while the parE gene-encoded product undergoes a single-site mutation (Thr333 → Ile) (threonine mutated to isoleucine)Abbreviated as T333I). Wherein parET333IIs the first reported new mutation site found in this study. This is the first discovery of the drug-resistant mutation site gyrA of quinolone drugsS83FAnd parET333IMeanwhile, the strain exists in a salmonella typhimurium strain, and the strain is not reported at home and abroad. Meanwhile, the strain does not have any plasmid-mediated quinolone drug resistance gene. Therefore, these mutation sites resulted in the Salmonella Sal103t strain exhibiting drug or intermediate resistance to quinolones, and the minimum inhibitory concentrations of nalidixic acid, ciprofloxacin, levofloxacin and moxifloxacin against this strain were 4096. mu.g/mL (resistant), 1. mu.g/mL (intermediate resistant), respectively. Therefore, the salmonella Sal103t can be used as a model material for screening functional microorganisms/novel antibacterial drugs, and has a good application prospect.
Drawings
FIG. 1 is a colony morphology of the Salmonella Sal103t strain of the present invention.
FIG. 2 is a morphological diagram of the Salmonella Sal103t strain according to the present invention under microscopic examination.
FIG. 3 is a schematic representation of the biochemical identification of API 20E of the Salmonella Sal103t strain of the present invention.
FIG. 4 shows the specific sites where the gyrA gene of Salmonella Sal103t of the present invention is mutated; gyrA (324C → T, Ser83 → Phe).
FIG. 5 shows the specific sites where the parE gene in Salmonella Sal103t of the present invention is mutated; parE (998C → T, Thr333 → Ile).
Detailed Description
To more clearly illustrate the technical solutions of the present invention, the following embodiments are further described, but the present invention is not limited thereto, and these embodiments are only some examples of the present invention.
EXAMPLE 1 isolation of the strains
The salmonella Sal103t is separated from fresh pork food in a certain supermarket in Shaokuan city, Guangdong province, China, and the collected sample is detected by referring to the food safety national standard food microbiology inspection salmonella GB 4789.4-2010. A sample of 25g (mL) was added to 225mL of Buffered Peptone Water (BPW) and homogenized in a sterile homogenization bag and incubated at 37 ℃ for 8-18 h. Gently shaking the cultured sample mixture, inoculating 1mL of the cultured sample mixture into 10mL of sodium sulfosate brilliant green (TTB) enrichment liquid, culturing at 42 ℃ for 18 h-24 h, and inoculating 1mL of the cultured sample mixture into 10mL of Selenite Cystine (SC) enrichment liquid, and culturing at 37 ℃ for 18 h-24 h. Taking enrichment liquid 1 ring by using an inoculating ring respectively, streaking and inoculating the enrichment liquid to a salmonella chromogenic medium plate, culturing the enrichment liquid for 18-24h at 37 ℃, and observing bacterial colonies growing on the plate. Typical salmonella colonies were purple, round, moist, flat-edged on the chromogenic plates (fig. 1). The target colonies were transferred from Nutrient Agar (NA) plates to brain heart infusion Broth (BHI) and incubated overnight at 37 ℃. The bacterial solution was aseptically added to a glycerin tube having a final concentration of 50%, stored in a refrigerator at-40 ℃ and freeze-dried in a tube, to thereby obtain the strain Sal103 t.
Example 2 Strain identification and culture
The purified strain Sal103t is subjected to morphological characteristics, physiological and biochemical characteristics, serotype identification and the like.
And (3) dyeing microscopic examination: the colonies were smeared, gram stained, and the morphology was visualized by microscopic examination. Salmonella was gram-negative, short rod-shaped (fig. 2).
API 20E identification: individual colonies were scraped from the NA plate, prepared as a cell suspension of appropriate turbidity with physiological saline, and characterized using an API 20E biochemical identification reagent strip (FIG. 3).
Serotype identification: serotyping the separated strain of salmonella by adopting a slide agglutination method. Checking the thallus antigen (O antigen) of salmonella, then determining the I phase and II phase flagellum antigen (H antigen) of the strain in turn, and finally referring to the salmonella diagnosis antigen table to make serotype diagnosis.
The result shows that the strain Sal103t is gram-negative, Brevibacterium, and API 20E is identified as Salmonella, and the coincidence rate is 89.4%. The biochemical characteristics are as follows: arginine double hydrolase, lysine decarboxylase and ornithine decarboxylase are positive, citrate is decomposed, hydrogen sulfide is generated, glucose, mannitol, inositol, sorbitol, rhamnose, melibiose, arabinose, urease, phenylalanine deaminase, oxidase, ONPG tests, indole tests and VP tests are negative, gelatin cannot be liquefied, cane sugar and amygdalin are not fermented, and the salmonella typical physiological and biochemical characteristics are provided. The serum antigen is identified as 1,4,5,12: i:1,2, a typical serotype of Salmonella typhimurium.
The appearance, morphology, gram stain, biochemical reaction and serological reaction of the strain Sal103t are comprehensively judged, so that the strain Sal103t is identified as Salmonella enterica subsp.
Salmonella sali 103t was deposited at the guangdong province collection of microorganisms (GDMCC) at 27/10/2020, address: guangzhou city, Jielizhou 100 large yard, building 59, building 5, the number of deposit is: GDMCC No: 61247.
example 3 drug sensitivity profiling
The broth dilution method was used to examine the level of resistance of the Salmonella Sal103t strain to quinolone drugs, including nalidixic acid, ciprofloxacin, levofloxacin and moxifloxacin, according to the methods and results criteria of the American Clinical Laboratory Standards Institute (CLSI) version 2018. Inoculating Salmonella Sal103t strain in a test tube containing 4 mM MH broth, incubating to logarithmic phase, and turbidifying with 0.5M turbidimetric tube to adjust the concentration of the broth to 1 × 107About cfu/mL. Taking the bacterial liquid, and mixing the bacterial liquid with fresh MH broth according to the weight ratio of 1: diluting with 200 deg.C, and mixing. Selecting a sterile 96-hole flat-bottom microplate, adding 100 mu L of MH broth culture medium into the 1 st hole of each row, then adding 100 mu L of the drug to be detected into the 1 st hole of each row, uniformly mixing, taking out 100 mu L, moving to the 2 nd hole, and repeating the steps until the 12 th hole is uniformly mixed, sucking 100 mu L and discarding. Finally, the mixed Salmonella Sal103t (1X 10) of the invention is added into each hole5cfu/mL) of the suspension. Selecting a proper hole site and adding 200 mu L MH broth culture medium as a negative control; as positive controls, 100. mu.L of Salmonella Sal103t broth and 100. mu.L of MH broth were added. Each group was run in parallel 3 times. The culture plate is placed in an incubator at 37 ℃ for 18-20h to observe the result, and the MIC value of the culture plate is determined. The culture plate is taken out from the incubator,and (4) reading the OD value by using a microplate reader to obtain a drug sensitivity result and an analysis report. Or judging the positive and negative results of each hole by naked eyes, wherein the turbidity is positive and the clarity is negative. Through experiments, the Minimum Inhibitory Concentrations (MICs) of nalidixic acid, ciprofloxacin, levofloxacin and moxifloxacin on salmonella Sal103t strain are 4096 mu g/mL (drug resistance), 1 mu g/mL (intermediate drug resistance) and 1 mu g/mL (intermediate drug resistance), respectively. The results of the specific drug sensitivity of the Salmonella Sal103t strain are shown in Table 1.
TABLE 1 resistance of Salmonella Sal103t strain to quinolone drugs
Example 4 detection of quinolone drug resistance Gene of Salmonella Sal103t Strain
(1) Whole genome second-generation sequencing and Blast sequence analysis target drug resistance gene
Whole genome secondary sequencing was performed on the Salmonella Sal103t strain. The genomic DNA was fragmented into 400bp fragments using a Covaris M200 sonicator and Library construction was performed using the Ion Plus Fragment Library Kit. Whole genome sequencing was performed using an Ion Torrent S5 sequencer. Genomic de novo assembly was performed by SPAdes v3.6.2. Meanwhile, the genome sequence after splicing is subjected to genome component prediction such as coding gene, tRNA, rRNA and the like by using prokka, and coding gene function annotation is carried out.
And detecting the drug resistance gene of the quinolone drugs by using a local Blast sequence analysis technology. Local Blast analysis: constructing a local database db.nt (103t.ffn is a name of a strain whole genome sequencing sequence information document and 103tdb.nt is a constructed database name) by using a whole genome sequencing sequence information document of a salmonella strain Sal103t by using an operation command ' makeblastdb-in 103t.ffn-dbtype nucleus-pars _ seqids-out 103tdb.nt ', and then targeting sequence information documents of target sites (QRD genes: gyrA, gyrB, parC, parE; PMQR genes: qnrA, qnrB, qnrC, qnrD, qnrS, qnrVC, aac (6') -Ib-cr, qep A, qoxAB) (taking analysis of the sequence information documents of the gyrA genes as examples and the gyrA.txt as target genes), blast analysis of the gyrA gene (outgyrAresult is the name of the output result) is performed on the local database 103tdb.nt by the operation command "blastn-db 103tdb.nt-evalue 1e-5-outfmt 0-num _ descriptions 10-num _ threads 64-query gyra.txt-outgyrAresult". And finally, checking the output file and judging the result.
(2) PCR amplification target drug resistance gene and Sanger method first generation sequencing analysis
The detection results of the target drug resistance gene of the whole genome sequencing and Blast sequence analysis of the strain are further verified by adopting a PCR amplification and first-generation sequencing analysis method. And designing a PCR amplification primer of the target drug-resistant gene according to the published gene sequence, wherein the primer sequence is shown in Table 2. The PCR amplification reaction system (25. mu.L) was performed by single PCR: 12.5 mu L of 2 XPremix Taq, 120nmol/L of each of the upstream primer and the downstream primer, 2 mu L of template and complete ultra-pure water; PCR amplification reaction conditions: pre-denaturation at 95 ℃ for 5 min; denaturation at 95 ℃ for 45s, annealing at 55-60 ℃ for 45s, and extension at 72 ℃ for 45s for 30 cycles; extension at 72 ℃ for 10 min. And carrying out electrophoresis detection (120V, 30min), purification and first-generation sequencing analysis on the amplified fragment to obtain the sequence information of the amplified target gene. Performing Blast analysis and result judgment on the sequencing result. The PCR amplification primers for each target gene designed in the research are as follows:
TABLE 2 Salmonella gyrA and parE primers and amplified fragment sizes
(3) Detection result of quinolone drug resistance gene of salmonella Sal103t strain
Comparing with a salmonella typhimurium standard strain LT2 gene sequence, and detecting mutations of gyrA and parE in QRDR genes of a salmonella Sal103t strain; the gyrA gene coding product is subjected to single-site mutation: ser83 → Phe (serine mutated to phenylalanine, abbreviated S83F), while the parE gene-encoded product undergoes a single-site mutation (Thr333 → Ile) (threonine mutated to isoleucine, abbreviated T333I). Whereas PMQR genes (qnrA, qnrB, qnrC, qnrD, qnrS, qnrVC, aac (6') -Ib-cr, qepA, oxqAB) were all not detected. And further verifying by adopting PCR amplification and Sanger method generation sequencing, wherein the detection results of the two methods are consistent.
The amino acid symbols have the meaning: serine (Serine, abbreviated to Ser or S), Phenylalanine (Phenylalanine, abbreviated to Phe or F), Threonine (Threonine, abbreviated to Thr or T), Isoleucine (Isoleucine, abbreviated to Ile or I).
Details of the gyrA and parE mutations detected in QRDR gene of Salmonella Sal103t strain are shown in FIGS. 4-5.
Example 5 application of quinolone resistant Salmonella Sal103t Strain
The specific application method of the salmonella Sal103t is mainly embodied in two aspects:
(1) the salmonella Sal103t simultaneously carries two quinolone drug resistance mutation sites gyrAS83FAnd parET333IThe drug resistance and intermediate drug resistance to four quinolone antibacterial drugs (nalidixic acid, ciprofloxacin, levofloxacin and moxifloxacin), particularly ciprofloxacin, are considered as good drugs for treating infection caused by salmonella, and the effect is always remarkable in recent years. Wherein parET333IIs the first reported new mutation site found in this study. The drug-resistant mutation site gyrA of the quinolone drugs is also found for the first timeS83FAnd parET333IMeanwhile, the strain exists in a salmonella typhimurium strain, and the strain is not reported at home and abroad. The occurrence of multiple mutations of quinolone drug resistance determining region genes in the same strain has important guiding significance for discussing the drug resistance generating mechanism of quinolone drugs and researching new strategies for treating salmonella infection, and can be used as an important material for seeking a bacterial drug resistance mechanism.
(2) The salmonella Sal103t simultaneously carries two quinolone drug resistance mutation sites gyrAS83FAnd parET333IFour quinolone antibacterial drugs (nalidixic acid, ciprofloxacin, levofloxacin)And moxifloxacin) to produce drug resistance and intermediate drug resistance, and can also be used as an important model strain for screening novel antibacterial drugs.
The screening method of the antibacterial drug comprises the following steps:
precisely weighing a certain amount of the drug to be detected, dissolving the drug with a proper solvent, and preparing the drug into a solution of 30mg/mL for later use. Then, a solution with a concentration of 2000. mu.g/mL is prepared by using a suitable solvent, and the solution is used for determining the Minimum Inhibitory Concentration (MIC) and the Minimum Bactericidal Concentration (MBC) and is stored in a refrigerator at-40 ℃ for later use. One day before the experiment, the salmonella Sal103t of the invention stored in a refrigerator at-40 ℃ is taken out, and after the salmonella is placed at room temperature, a small amount of bacterial colonies are respectively picked by inoculating loops, and are respectively inoculated on MH agar culture media, and are cultured in a constant temperature box at 37 ℃ for 18-24 h. On MH agar medium with newly grown colony, selecting small amount of activated colony with inoculating loop, diluting with sterile normal saline to prepare 1 × 107cfu/mL of bacterial suspension (standard turbiditube control) for use.
Screening the drug to be tested which is sensitive to the drug-resistant strain, and determining the sensitivity of the strain to different drugs according to the paper diffusion method recommended by the American Clinical Laboratory Standards Institute (CLSI). Dipping prepared standard bacteria with a concentration of 1 × 10 with a sterile cotton swab7cfu/mL of bacterial suspension, using a coating rod to evenly spread to the corresponding MH agar medium. Placing the sterilized Oxford cup on the coated MH agar culture medium, and adding 50 microliter of prepared liquid medicine (30mg/mL) into the Oxford cup by using a micropipettor; culturing in 37 deg.C incubator for 18-24 hr, and measuring the size of antibacterial zone. An equal volume of MH broth was taken and vehicle was added as a blank control.
Drugs to be tested sensitive to drug-resistant strains were screened, and MIC values of the strains to different drugs were determined according to broth dilution method recommended by the american Clinical Laboratory Standards Institute (CLSI), and each group was operated 3 times in parallel to obtain concentration average values. When determining MIC value, 1X 10 of the total amount of the mixture was mixed with sterile physiological saline7The bacterial suspension was further diluted to a concentration of 1X 10 cfu/mL5cfu/mL. Sterile 96-well flat-bottom microplates were selected, 100 μ L of MH broth was added to well 1 of each row, followed by well 1 of each rowAdding 100 mu L of the drug to be detected, mixing uniformly, taking out 100 mu L, moving to the 2 nd hole, repeating the steps until the 12 th hole is mixed uniformly, sucking 100 mu L, and discarding. Finally, the mixed Salmonella Sal103t (1X 10) of the invention is added into each hole5cfu/mL) of the suspension. Selecting a proper hole site and adding 200 mu L MH broth culture medium as a negative control; as positive controls, 100. mu.L of Salmonella Sal103t broth and 100. mu.L of MH broth were added. The culture plate is placed in an incubator at 37 ℃ for 18-20h to observe the result, and the MIC value of the culture plate is determined. And taking out the culture plate from the incubator, and reading the OD value by using an enzyme-labeling instrument to obtain a drug sensitivity result and an analysis report. Or judging the positive and negative results of each hole by naked eyes, wherein the turbidity is positive and the clarity is negative. When the MIC value is determined, the mixture of bacteria and MH broth culture medium 3-5 holes before the MIC value is taken and inoculated on MH agar medium, the mixture is placed in an incubator at 37 ℃, the mixture is taken out after being cultured for 22h and observed, and the lowest drug concentration with the average value less than 5 is determined as the MBC of the compound.
Finally, it should be noted that the above embodiments are only used for illustrating the technical solutions of the present invention and not for limiting the protection scope of the present invention, and although the present invention is described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions can be made on the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.
SEQUENCE LISTING
<110> institute of microbiology, academy of sciences of Guangdong province (center for microbiological analysis and detection of Guangdong province)
<120> Salmonella typhimurium with quinolone drug resistance genes gyrA and parE mutated simultaneously and application thereof
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<210> 4
<211> 20
<212> DNA
<213> Artificial Synthesis
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Claims (3)
1. A Salmonella typhimurium characterized in that it is Salmonella typhimurium (I)Salmonella enterica subsp. enterica serovar Typhimurium) Sal103t, which is classified and named as Salmonella Typhimurium (A)Salmonella enterica subsp. enterica serovar Typhimurium), which has been deposited at 27.10.2020 in the collection of microorganisms of the Guangdong province, at the deposition site: guangzhou city, Jielizhou 100 large yard, building 59, floor 5, the preservation number: GDMCC No: 61247.
2. a microbial agent, comprising the salmonella typhimurium of claim 1.
3. Use of the salmonella typhimurium of claim 1 as a model strain for screening novel antibacterial agents.
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