Development of a novel real-time PCR multiplex assay for detection of Streptococcus equi subspecies equi and Streptococcus equi subspecies zooepidemicus.

Strangles is a contagious bacterial disease of horses caused by Streptococcus equi subspecies equi (SEE) that occurs globally. Rapid and accurate identification of infected horses is essential for controlling strangles. Because of limitations of existing PCR assays for SEE, we sought to identify novel primers and probes that enable simultaneous detection and differentiation of infection with SEE and S. equi subsp. zooepidemicus (SEZ). Comparative genomics of U.S. strains of SEE and SEZ (n = 50 each) identified SE00768 from SEE and comB from SEZ as target genes. Primers and probes for real-time PCR (rtPCR) were designed for these genes and then aligned in silico with the genomes of strains of SEE (n = 725) and SEZ (n = 343). Additionally, the sensitivity and specificity relative to microbiologic culture were compared between 85 samples submitted to an accredited veterinary medical diagnostic laboratory. The respective primer and probe sets aligned with 99.7 % (723/725) isolates of SEE and 97.1 % (333/343) of SEZ. Of 85 diagnostic samples, 20 of 21 (95.2 %) SEE and 22 of 23 SEZ (95.6 %) culture-positive samples were positive by rtPCR for SEE and SEZ, respectively. Both SEE (n = 2) and SEZ (n = 3) were identified by rtPCR among 32 culture-negative samples. Results were rtPCR-positive for both SEE and SEZ in 21 of 44 (47.7 %) samples that were culture-positive for SEE or SEZ. The primers and probe sets reported here reliably detect SEE and SEZ from Europe and the U.S., and permit detection of concurrent infection with both subspecies.

Effects of chlorantraniliprole insecticide on innate immune response of silver catfish (Rhamdia quelen) naturally infected with Aeromonas hydrophila.

It is well documented today that pesticides, used in crop production, may modulate the immune system of healthy fish. However, there is still only limited information regarding the effects of these anthropogenic stressors in conjunction with natural stressors (pathogens), on the innate immune responses of freshwater fish. Thus, the aim of this investigation was to compare the innate immune response of two groups of fish (Rhamdia quelen), naturally infected with Aeromonas hydrophila, exposed and unexposed to a non-lethal concentration of chlorantraniliprole (CAP) insecticide (0.0 and 1.3 µg/L/24 h). Unhealthy fish exposed to CAP showed significant higher total leukocyte counts and neutrophils percentage compared to non-exposed infected fish). However, the monocytes and eosinophils percentage significantly decreased in fish exposed to CAP. Furthermore, lysozyme activity values measured in plasma, skin mucus, gill and intestine significantly reduced in fish exposed to CAP. The CAP-induced immunomodulation may interfere on the ability of the animal to heal or fight the infection, and possible contribute to the spread of bacterial infection in fish production or environment.

Guinea pig infection with the intracellular pathogen Rhodococcus equi.

Rhodococcus equi is an opportunistic, intracellular pathogen that causes pyogranulomatous pneumonia in foals and immunocompromised people. Currently, there is no experimental model of R. equi pneumonia other than intra-bronchial experimental infection of foals with R. equi, which is labor-intensive and costly. This study's objective was to develop a guinea pig (GP) model of R. equi pneumonia that would facilitate development of novel approaches for controlling and preventing this disease. Guinea pigs were infected with either 101, 102, 103, or 104 colony forming units (CFUs) of a virulent strain of R. equi using a Madison aerosol chamber, or 106 or 107 CFUs of this strain intratracheally. Animals were monitored daily for clinical signs of pneumonia, and were euthanized and necropsied on days 1, 3, 7, or 35 post-infection (PI). Lung homogenates were plated onto selective agar to determine bacterial load. No clinical signs of disease were observed regardless of the inoculum dose or infection method. No bacteria were recovered from GPs euthanized at 35 days PI. Histology and immunostaining of T-cells, B-cells, and macrophages in lungs showed that inflammatory responses in infected GPs were similarly unremarkable irrespective of dose or route of infection. Guinea pigs appear to be resistant to pulmonary infection with virulent R. equi even at doses that reliably produce clinical pneumonia in foals.

The Essential Oil of Hyptis mutabilis in Ichthyophthirius multifiliis Infection and its Effect on Hematological, Biochemical, and Immunological Parameters in Silver Catfish, Rhamdia quelen.

This study evaluated the activity of leaf essential oil (EO) from Hyptis mutabilis as well as its major constituent, (-)-globulol, in infections by the parasite Ichthyophthirius multifiliis (ich). Effects on hematological, biochemical, and immunological parameters of silver catfish, Rhamdia quelen, exposed to the same samples also were evaluated. In the first experiment, naturally infected fish were treated with EO (0, 10, and 20 mg · L-1) and ethanol, using several methods of exposure. Fish mortality and the number of trophonts per fish were assessed after 48 and 96 hr. Hour-long daily baths resulted in optimal survival, so this methodology was used for the second experiment, in which infected animals were exposed to (-)-globulol at 2.5 and 5 mg · L-1. The most effective concentrations in Experiments 1 and 2 were chosen for Experiment 3, in which healthy animals were subjected to hour-long daily baths with EO (20 mg · L-1) or (-)-globulol (2.5 mg · L-1). Additionally, an in vitro experiment was performed with EO and globulol at the same concentrations of the in vivo test. EO and (-)-globulol increased the survival of fish infected with ich and altered certain hematological and biochemical parameters. After 4 days, levels of hematocrit, erythrocytes, and leukocytes increased significantly in healthy animals exposed to EO. Exposure to (-)-globulol increased leukocyte number alone. No significant differences in nonspecific immunological parameters were detected when treated groups were compared to controls, but the leukocytosis observed in EO- and globulol-treated healthy animals indicates that EO and (-)-globulol increased innate immunity in these fish. An in vitro antiparasitic effect was observed in both samples.

S-(+)- and R-(-)-linalool: a comparison of the in vitro anti-Aeromonas hydrophila activity and anesthetic properties in fish.

Linalool is the main compound of many essential oils and occurs in two isomeric forms: S-(+)- and R-(-)-linalool. This study aimed to determine if linalool isomers have different antimicrobial and anesthetic properties in fish. For this purpose, these compounds were previously isolated from Lippia alba (Mill.)N. E. Brown and Ocimum americanum L. essential oils. Antimicrobial effects were evaluated through the microdilution test against Aeromonas hydrophila, an important fish disease etiologic agent. Induction time until sedation, anesthesia and recovery time were determined in silver catfish (Rhamdia quelen) through bath exposure (60, 180, 300 or 500 µL L-1). The results showed different biological properties for the isomers being S-(+)-linalool the only active against A. hydrophila at 3.2 mg mL-1. The sedation was induced without differences between the compounds, however R-(-)-linalool promoted faster anesthesia. There were no differences regarding the recovery time of the animals exposed to the linalool isomers. Although both S-(+)- and R-(-)-linalool can be used for sedative purposes, their use in A. hydrophila infection is inadvisable due to the high effective concentration. Considering anesthesia as the main objective, the R-(-)-linalool demonstrated clear advantages at lower concentration.

S-(+)- and R-(-)-linalool: a comparison of the in vitro anti-Aeromonas hydrophila activity and anesthetic properties in fish

ABSTRACT Linalool is the main compound of many essential oils and occurs in two isomeric forms: S-(+)- and R-(-)-linalool. This study aimed to determine if linalool isomers have different antimicrobial and anesthetic properties in fish. For this purpose, these compounds were previously isolated from Lippia alba (Mill.)N. E. Brown and Ocimum americanum L. essential oils. Antimicrobial effects were evaluated through the microdilution test against Aeromonas hydrophila, an important fish disease etiologic agent. Induction time until sedation, anesthesia and recovery time were determined in silver catfish (Rhamdia quelen) through bath exposure (60, 180, 300 or 500 μL L-1). The results showed different biological properties for the isomers being S-(+)-linalool the only active against A. hydrophila at 3.2 mg mL-1. The sedation was induced without differences between the compounds, however R-(-)-linalool promoted faster anesthesia. There were no differences regarding the recovery time of the animals exposed to the linalool isomers. Although both S-(+)- and R-(-)-linalool can be used for sedative purposes, their use in A. hydrophila infection is inadvisable due to the high effective concentration. Considering anesthesia as the main objective, the R-(-)-linalool demonstrated clear advantages at lower concentration.

Chloroquine inhibits Rhodococcus equi replication in murine and foal alveolar macrophages by iron-starvation.

Rhodococcus equi preferentially infects macrophages causing pyogranulomatous pneumonia in young foals. Both the vapA and rhbC genes are up-regulated in an iron (Fe)-deprived environment, such as that found within macrophages. Chloroquine (CQ) is a drug widely used against malaria that suppresses the intracellular availability of Fe in eukaryotic cells. The main objective of this study was to evaluate the ability of CQ to inhibit replication of virulent R. equi within murine (J774A.1) and foal alveolar macrophages (AMs) and to verify whether the mechanism of inhibition could be Fe-deprivation-dependent. CQ effect on R. equi extracellular survival and toxicity to J774A.1 were evaluated. R. equi survival within J774A.1 and foal AMs was evaluated under CQ (10 and 20µM), bovine saturated transferrin (bHTF), and bovine unsaturated transferrin (bATF) exposure. To explore the action mechanism of CQ, the superoxide anion production, the lysozyme activity, as well as the relative mRNA expression of vapA and rhbC were examined. CQ at≤20µM had no effect on R. equi extracellular multiplication and J774A.1 viability. Exposure to CQ significantly and markedly reduced survival of R. equi within J774A.1 and foal AMs. Treatment with bHTF did not reverse CQ effect on R. equi. Exposure to CQ did not affected superoxide anion production or lysozyme activity, however vapA and rhbC expression was significantly increased. Our results reinforce the hypothesis that intracellular availability of Fe is required for R. equi survival, and our initial hypothesis that CQ can limit replication of R. equi in J774A.1 and foal AMs, most likely by Fe starvation.

Biofilm formation by Rhodococcus equi and putative association with macrolide resistance / Formação de biofilme por Rhodococcus equi e possível associação com resistência a macrolídeos

Rhodococcus equi is a facultative intracellular pathogen, which cause severe pyogranulomatous pneumonia in foals and tuberculosis-like lesions in humans. Its ability to form biofilm was described in strains isolated from chronic diseases associated to treatment failures in humans. This study aimed to verify the biofilm formation by 113 R. equi isolated from equine samples (clinical and fecal) using two different methods (biofilm-culturing with and without additional glucose and epifluorescence microscopy). We also aimed to determine the efficacy of azithromycin, clarithromycin and erythromycin on R. equi in established biofilm. We found 80.5% (26/41) and 63% (58/72) biofilm-positive isolates, in fecal and clinical samples, respectively. The additional glucose increased the biofilm formation by R. equi fecal samples, but not by clinical samples. The antimicrobials tested herein were not able to eradicate R. equi in biofilm even at higher concentrations. This is the first study showing the biofilm formation by R. equi isolated from equine samples. Our findings indicate that R. equi biofilm-producers may be more resistant to the antimicrobials evaluated. Further studies are warranted to test this hypothesis...

Rhodococcus equi é um patógeno intracelular facultativo, o qual causa pneumonia piogranulosa severa em potros e lesões semelhantes à tuberculose em humanos. A sua capacidade de formar biofilme foi descrita em cepas humanas, isoladas a partir de doenças crônicas associadas a falhas de tratamento. Este estudo teve como objetivo verificar a formação de biofilme por 113 cepas de R. equi, isoladas a partir de amostras de equinos (clínicas e fecais), utilizando-se dois diferentes métodos (biofilme em cultura - com e sem adição de glicose - e microscopia de epifluorescência). Além disso, buscou-se determinar a eficácia da azitromicina, claritromicina e eritromicina sobre biofilme consolidado de R. equi. Verificou-se 80,5% (26/41) e 63% dos isolados (58/72) positivos para formação de biofilme, em amostras fecais e clínicas, respectivamente. A adição de glicose amentou a formação de biofilme em amostras fecais, mas não em amostras clínicas. Os antimicrobianos aqui testados não foram capazes de erradicar R. equi em biofilme consolidado, mesmo em concentrações elevadas. Este é o primeiro estudo a demonstrar a formação de biofilme por cepas de R. equi isoladas a partir de amostras de equinos. Os resultados indicam que os isolados de R. equi produtores de biofilme podem ser mais resistentes aos antimicrobianos avaliados. Estudos adicionais são necessários para testar essa hipótese...

Differences in the antimicrobial susceptibility profiles of Moraxella bovis, M. bovoculi and M. ovis.

The aim of this study was to determine the differences in the antimicrobial susceptibility profiles of Moraxella bovis, M. bovoculi and M. ovis. Thirty-two strains of Moraxella spp. isolated from cattle and sheep with infectious keratoconjunctivitis were tested via broth microdilution method to determine their susceptibility to ampicillin, cefoperazone, ceftiofur, cloxacillin, enrofloxacin, florfenicol, gentamicin, neomycin, oxytetracycline and penicillin. The results demonstrated that Moraxella spp. strains could be considered sensitive for most of the antimicrobials tested in this study, but differences between the antimicrobial susceptibility profiles of these three Moraxella species were found. M. bovis might differ from other species due to the higher MIC and MBC values it presented.

Differences in the antimicrobial susceptibility profiles of Moraxella bovis, M. bovoculi and M. ovis

The aim of this study was to determine the differences in the antimicrobial susceptibility profiles of Moraxella bovis, M. bovoculi and M. ovis. Thirty-two strains of Moraxella spp. isolated from cattle and sheep with infectious keratoconjunctivitis were tested via broth microdilution method to determine their susceptibility to ampicillin, cefoperazone, ceftiofur, cloxacillin, enrofloxacin, florfenicol, gentamicin, neomycin, oxytetracycline and penicillin. The results demonstrated that Moraxella spp. strains could be considered sensitive for most of the antimicrobials tested in this study, but differences between the antimicrobial susceptibility profiles of these three Moraxella species were found. M. bovis might differ from other species due to the higher MIC and MBC values it presented.

Lippia alba essential oil promotes survival of silver catfish (Rhamdia quelen) infected with Aeromonas sp.

In vitro and in vivo activity of the Lippia alba essential oil (EO) against Aeromonas sp. was evaluated. In the in vitro assay the minimum inhibitory concentration (MIC) and a minimum bactericidal concentration (MBC) of EO for Aeromonas cells were determined using the microdilution method. Twenty five strains of Aeromonas sp. isolated from infected fish obtained from local fish farms were used. MIC and MBC values were 2862 and 5998 µg mL-1 for L. alba EO and 0.5 and 1.2 µg mL-1 for gentamicin, respectively. In the in vivo assay silver catfish juveniles (Rhamdia quelen) (7.50 ± 1.85 g and 10.0 ± 1.0 cm) with typical injuries associated to Aeromonas infection were divided into four treatments (in triplicate n=10): untreated fish (negative control), 10 mg L-1 of gentamicin, and 20 or 50 µL L-1 of EO. Fish were maintained in aerated 20 L plastic boxes. After 10 days survival of silver catfish infected with Aermonas sp. and treated with essential oil (50 µL L-1) was greater than 90%.

Ação dos desinfetantes sobre a adesão e biofilme consolidado de staphylococcus spp / Action of products based on chlorhexidine and iodine for the adhesion and consolidated biofilm of staphylococcus spp isolated from milk

Staphylococcus spp. são reconhecidos como importantes causadores de mastites em rebanhos leiteiros. Esses micro-organismos têm a capacidade de produzir uma estrutura denominada biofilme, que é responsável pela sobrevivência e muitas vezes pela resistência a ação de produtos desinfetantes e as demais condições adversas. Neste trabalho avaliou-se a ação de dois produtos pós-dipping a base de iodo (0,7%) e clorexidine (2,0%) sobre a adesão de Staphylococcus coagulase positiva (SCP) e Staphy-lococcus coagulase negativa (SCN) isolados de casos de mastite subclínica, e também sobre biofilmes pré formados a partir destes isolados. Os produtos testados apresentaram uma alta redução na taxa de adesão de todos os isolados. No entanto, a ação sobre os biofilmes consolidados só foi estatisticamente significativa sobre os SCN. Assim, ressalta-se a importância dos programas sanitários a fim de prevenir a formação de biofilmes e diminuir as fontes de contaminação da glândula mamaria em sistemas de produção leiteira.

Staphylococcus spp. are recognized as important causes of mastitis in dairy herds. These microorganisms have the ability to produce a structure called biofilm, which is responsible for the survival and often for resistance to the action of disinfectants and other harsh conditions. In this paper we evaluate the action of two teat dip products based on iodine (0,7%) and chlorhexidine (2,0%) on the adhesion of coagulase-positive staphylococci (CPS) and coagulase negative staphylococci (CNS) from subclinical mastitis cases, and also on its consolidate biofilm. The products tested showed a high reduction on the adhesion of all isolates. However, action on the consolidate biofilm was significant for CNS only, and unsatisfactory for CPS. Thus, it emphasizes the importance of health programs to prevent biofilm formation and reduce the sources of contamination of the mammary gland in dairy production systems.

The use of nitazoxanide against the pathogens Ichthyophthirius multifiliis and Aeromonas hydrophila in silver catfish (Rhamdia quelen).

The antiparasitic and antibacterial activities of nitazoxanide (NTZ) against Ichthyophthirius multifiliis (Ich) and Aeromonas hydrophila, respectively, were evaluated in silver catfish juveniles (Rhamdia quelen). In the first experiment, fish naturally infected by Ich were treated with different concentrations of NTZ. Fish mortality and the number of trophonts/juveniles were evaluated. In the second experiment, the antimicrobial potential in vitro (MIC test) and preventive potential in vivo against A. hydrophila was evaluated. The treatment with 1.5mgL(-1) NTZ showed the best result against Ich, with a significant reduction of trophonts and fish mortality of 10%. NTZ did not show in vitro activity against A. hydrophila at concentrations tested, but showed a probable preventive activity in vivo. However, these results are preliminary and more studies should be conducted.

Suscetibilidade a desinfetantes e perfil de resistência a antimicrobianos em isolados de Escherichia coli / Susceptibility to disinfectants and antimicrobial resistance profile in Escherichia coli isolates

A colibacilose, causada por Escherichia coli, é a enfermidade entérica de maior impacto na produção de suínos, podendo levar à morte do animal. Esta bactéria possui grande capacidade de desenvolver resistência a múltiplos antimicrobianos e a desinfetantes. Desta forma, estudos que abordem mecanismos de resistência e perfil de amostras de campo tornam-se necessários. E. coli é amplamente utilizada como modelo de estudos que exploram a resistência intrínseca e extrínseca a multidrogas. Neste trabalho, buscou-se verificar o perfil de sensibilidade de 62 isolados de E. coli de suínos frente a três desinfetantes e a 13 antimicrobianos. Ainda, em 31 destes isolados foi pesquisada a presença de mecanismo de efluxo. Dos três desinfetantes avaliados, o cloreto de alquil dimetil benzil amônio+poliexietilenonilfenileter foi o que se mostrou mais eficaz (100%), seguido do glutaraldeído+cloreto de alquil dimetil benzil amônio (95,2%) e do cloreto de alquil dimetil benzil amônio (88,8%). Dentre os antimicrobianos testados, observou-se maior resistência para a tetraciclina (62,2%) e maior sensibilidade para o florfenicol (88,6%). A alta sensibilidade dos isolados frente aos desinfetantes pode estar relacionada à ausência de mecanismo de efluxo. O índice de resistência múltipla médio aos antimicrobianos foi de 0,52, o que demonstra um perfil multirresistente dos isolados, conduzindo para a necessidade do uso racional destas drogas em suinocultura.

Colibacillosis caused by Escherichia coli is the most important enteric disease in pig production, which may lead to death of the affected animal. The bacterium has a great ability to develop resistance to multiple antibiotics and disinfectants. Thus, investigation addressing mechanisms of resistance and profile of field samples is necessary. E. coli is widely used as a model for studies that explore the intrinsic and extrinsic resistance to multidrugs. In this paper, we attempt to associate the susceptibility profile of 62 isolates of E. coli to three disinfectants and 13 antimicrobials. Also 31 isolates were tested for the presence of efflux mechanism. Of the three disinfectants tested, alkyldimethylbenzylammonium chloride+nonyl phenoxy polyethoxy ethanol was the most effective (100%), followed by glutaraldehyde+alkyldimethylbenzylammonium chloride (95.2%) and alkyldimethylbenzylammonium chloride (88.8%). Among the antimicrobials tested, there was greater resistance to tetracycline (62.2%) and higher sensitivity to florfenicol (88.6%). The high sensitivity of the isolates against disinfectants may be related to the absence of efflux mechanism. The average index of multiple resistance to antimicrobials was 0.52, what demonstrates a profile of multidrug resistant isolates, showing the need for rational use of these drugs in pig production.

Susceptibility of Trypanosoma evansi to propolis extract in vitro and in experimentally infected rats.

Current therapy of Trypanosoma evansi infections is not effective for the vast majority of animals with relapsing parasitemia and clinical signs. Recently, attention is being focused on the antiparasitic activity of propolis. This study evaluated the susceptibility of T. evansi to propolis extract in vitro and in vivo. A dose-dependent trypanocidal activity of propolis extract was observed in vitro. All trypomastigotes were killed 1 h after incubation with 10 µg mL(-1) of the extract. In vivo, the concentrations of 100, 200, 300 and 400 mg kg(-1) administered orally for 10 consecutive days showed no curative effect, and the rats died from the disease. However, rats treated with the two highest concentrations of propolis extract showed higher longevity than the other groups. Based on these data, we concluded that T. evansi is susceptible to propolis in vitro. Despite the lack of curative efficacy observed in vivo at the concentrations tested, the propolis extract can prolong life in rats infected with the protozoan.

Perfil fenotípico e susceptibilidade antimicrobiana de Streptococcus equi isolados de equinos da região Sul do Brasil / Phenotypic profile and antimicrobial susceptibility of Streptococcus equi isolated from horses in southern Brazil

As características fenotípicas [morfológicas, bioquímicas, susceptibilidade aos antimicrobianos, índice de resistência múltipla aos antimicrobianos (IRMA), concentração inibitória mínima (CIM) e concentração bactericida mínima (CBM) da benzilpenicilina] de 38 isolados de Streptococcus equi oriundos de amostras clínicas de animais com adenite equina foram alvo deste estudo. A fenotipia demonstrou três padrões de colônias, três biotipos de fermentação de carboidratos e variação de 0 a 0,4 no IRMA. Todos os isolados de S. equi demonstraram sensibilidade à penicilina, tanto pelo método de disco difusão quanto pelo método de microdiluição. A CIM e CBM média de benzilpenicilina foi de 0,0095μg/mL e 0,0267μg/mL para S. equi subesp. equi e de 0,0128μg/mL e 0,0380μg/mL para S. equi subesp. zooepidemicus. Os valores de CIM e CBM diferiram entre as subespécies (p<0,05). O diâmetro do halo de inibição de penicilina demonstrou relação com a CIM (ì=0,03638 - 0,00072x) para S. equi subesp. equi. Também foi demonstrada relação entre o diâmetro do halo de inibição de penicilina com a CBM para S. equi subesp. equi (ì=0,10931- 0,00223x). Entretanto para as amostras de S. equi subesp. zooepidemicus esta relação somente foi verificada para a CBM (ì=0,1322 - 0,00271x). A CIM de benzilpenicilina frente às amostras isoladas da região Central, Planalto e Sul do estado do Rio Grande do Sul foram estatisticamente semelhantes, mas diferiram do isolado do estado do Paraná, sugerindo o caráter atípico desta cepa. Todos os isolados de S. equi são sensíveis à penicilina e sulfazotrim, confirmando a eleição destes antimicrobianos para o tratamento das infecções por este agente na clínica veterinária. Os resultados obtidos não dispensam a utilização prudente dos antimicrobianos.

Phenotypic characteristics [morphology, biochemical fermentation, antimicrobial susceptibility, index of multiple resistances to antimicrobials (IMRA), minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of benzilpenicillin] of 38 Streptococcus equi isolates from clinical samples of horses with strangles were the aim of this study. The phenotypic analyses demonstrated three colony patterns, three carbohydrate fermentation biotypes and IMRA variation from 0 to 0.4. All the isolates of S. equi demonstrated sensitivity to penicillin, both by the disc diffusion method and microdilution method. The average MIC and MBC for benzilpenincillin were of 0.0095μg/mL and 0.0267μg/mL for S. equi subsp. equi and of 0.0128μg/mL and 0.0380μg/mL for S. equi subsp. zooepidemicus. The values of MIC and MBC differed between the subspecies (p<0.05). The diameter of penicillin inhibition halo demonstrated a relation with the MIC (ì=0.03638 - 0.00072x) for Streptococcus equi subsp. equi. A relation between the diameter of the inhibition halo of penincillin was also observed with the MBC for S. equi subsp. equi (ì=0.10931 - 0.00223x). However for the samples of S. equi subsp. zooepidemicus this relation was only verified with the MBC (ì=0.1322 - 0.00271x). The MIC of benzilpenicillin of the samples isolated from the Central, Planalto and South regions of Rio Grande do Sul were statistically similar, although different from the Paraná state sample, suggesting the atypical character of this strain. All the S. equi isolates are sensitive to penicillin and sulfazotrim, confirming these as antibiotics of choice for the treatment of infections caused by this agent in the clinical veterinary practice. The results obtained do not discard the prudent use of antimicrobials.