Revista Médica de la Universidad de Costa Rica. Volumen 2, número 2, artículo 8, septiembre 2008. http://www.revistamedica.ucr.ac. Artículo de Revisión Oxytetracycline and gentamicin: two clinically- relevant antimicrobials widely used by Costa Rican farmers. Implications of their use outside clinical settings and request for action. César Rodríguez Sánchez Sección de Bacteriología General and Centro de Investigación en Enfermedades Tropicales. Facultad de Microbiología. Universidad de Costa Rica. Resumen Esta mini-revisión se relaciona con el uso de la oxitetraciclina y la gentamicina, dos antimicrobianos de aplicación en medicina humana, en la horticultura y otras actividades productivas. El manuscrito inicia con una breve descripción del mecanismo de acción y uso clínico de ambas drogas. Seguidamente, el autor aborda los principales mecanismos de resistencia adquirida que han sido descritos hasta la fecha y algunas implicaciones de su mal uso y abuso en escenarios no hospitalarios. En este sentido, se presentan reportes de la ocurrencia de bacterias resistentes y genes de resistencia que demuestran la existencia de un riesgo y que justifican la instauración de programas de monitoreo y de impacto ambiental. Palabras clave: Oxitetraciclina, gentamicina, resistencia antimicrobiana, agricultura. Recibido: Marzo 2008. Aprobado: Abril 2008. Publicado: Septiembre 2008. __________________________________________________________________________ Revista electrónica publicada por el Departamento de Farmacología de la Escuela de Medicina de la Universidad de Costa Rica, 2060 San José, Costa Rica. ® All rights reserved. Revista Médica de la Universidad de Costa Rica. Volumen 2, número 2, artículo 8, septiembre 2008. Abstract This mini-review deals with oxytetracycline and gentamicin: two clinically-relevant antimicrobials that find extensive usage in horticulture and other productive activities. The manuscript begins with a few words on their mechanism of action and clinical utility. Thereafter I address the most important mechanisms of acquired or intrinsic resistance described to date and some implications of the unrestricted use of these drugs outside clinical settings. A number of reports on the occurrence of antimicrobial-resistance genes and antimicrobial-resistant bacteria in agroecosystems illustrates that a threat exists and justifies the plea for monitoring programs and environmental risk assessments with which the article concludes. Keywords: oxytetracycline, gentamicin, antimicrobial-resistance, plant agriculture. Mechanism of action and clinical utility of oxytetracycline and gentamicin Sobin et al. [1] were the first to purify atypical pneumonia syndromes, oxytetracycline from fermentation broths chlamydial genital diseases, Lyme inoculated with Streptomyces rimosus. disease, ehrlichiosis, and infections by This drug belongs to the tetracyclines, a Mycoplasma sp. [4]. Moreover, group of broad-spectrum antimicrobials oxytetracycline is useful for the topical that inhibit protein synthesis by blocking treatment of ocular disorders [5], the binding of aminoacyl-t-RNAs to the rosacea, and facial acne [6]. This group A-site of the ribosome [2]. In Gram- of antibiotics is contraindicated during negative bacteria, tetracyclines move pregnancy and in the childhood due to passively through membranes via porin adverse side effects such as dental channels and accumulate in the staining and interference with bone periplasmic space. Later on, their growth. In addition, the tetracyclines may movement across the cytoplasmic instigate photosensitivity as well as drug membrane requires energy provided by and food interactions that disturb normal the ∆pH of the proton motive force [3]. absorption processes in the gastrointestinal tract. The tetracyclines exhibit bacteriostatic activity against a wide variety of bacteria Gentamicin was first isolated from a and certain protozoans (Entamoeba strain of Micromonospora purpurea in histolytica, Giardia lamblia, Leishmania 1963 [7]. At concentrations below 2 µg major, Trichomonas vaginalis, and ml-1, it inhibits the initiation step of Toxoplasma gondii). They are protein synthesis through attachment to particularly helpful in the treatment of a conserved rRNA sequence located __________________________________________________________________________ Revista electrónica publicada por el Departamento de Farmacología de la Escuela de Medicina de la Universidad de Costa Rica, 2060 San José, Costa Rica. ® All rights reserved. Revista Médica de la Universidad de Costa Rica. Volumen 2, número 2, artículo 8, septiembre 2008. near the spot of codon-anticodon Mechanism of resistance to recognition in the A-site of the ribosome oxytetracycline and gentamicin [8]. At higher concentrations, it induces in the bacterial world misreading and accumulation of abnormally long polypeptides due to destabilizing effects on tRNA-mRNA Until now, four different mechanisms interactions [9]. Passage of gentamicin confer acquired resistance against the across the outer membrane of Gram- tetracyclines [17]. Namely, reduction of negative bacteria involves passive their intracellular concentration by disruption of Mg+2 bridges between means of energy-dependent efflux adjacent lipopolysaccharide molecules. proteins of the major facilitator Thereafter, its transport across superfamily (efflux proteins; EP), cytoplasmic membranes depends on protection of the ribosome by soluble electron transport. Penetration through proteins from the superfamily of porin channels is unlikely because of GTPases (ribosomal protection proteins; their large size [10]. RPP), enzymatic modification of the antibiotics in the presence of oxygen and NADPH (tetracycline modifying The aminoglycosides display enzymes; TME), and a further, poorly concentration-dependent, bactericidal understood, mechanism. The 23 EP action against a wide range of aerobic known so far, which catalyze the Gram-negative bacilli, staphylococci, exchange of a proton for a tetracycline- and certain Actinobacteria [11]. cation complex against a concentration Consequently, their commercialization gradient, consist of 12 (Gram-negative rapidly revolutionized the treatment of bacteria) or 14 (Gram-positive bacteria) Mycobacterium tuberculosis [12] and membrane-spanning helixes separated Pseudomonas aeruginosa [13]. These by short regions of hydrophilic amino antimicrobials exhibit some acids [18]. Intriguingly, 16 genes from advantageous features, such as poor this family of resistance determinants gastrointestinal absorption, reduced (~70%) are restricted to Gram-negative penetration into the cerebrospinal fluid, bacteria [17]. The second most negligible binding to plasma proteins, prevalent mechanism of acquired and fast excretion by uncompromised resistance to tetracyclines is due to kidneys [14]. However, they should be RPP. Considering their structural prescribed with caution because of their resemblance to the elongation factors recognized oto- and nephrotoxicity [15]. EF-Tu and EF-G, RPP seem to cause From this large family of antimicrobial an allosteric disruption that releases agents, the low cost and reliable activity tetracyclines from their binding sites and of gentamicin give good reasons for its returns the ribosome to its standard prescription [16]. post-translocational conformation [19]. Most genes for RPP have G+C% contents compatible with a Gram- positive origin. However, they are found in both Gram-positive and Gram- negative bacteria [17]. __________________________________________________________________________ Revista electrónica publicada por el Departamento de Farmacología de la Escuela de Medicina de la Universidad de Costa Rica, 2060 San José, Costa Rica. ® All rights reserved. Revista Médica de la Universidad de Costa Rica. Volumen 2, número 2, artículo 8, septiembre 2008. Besides the genetic elements discussed Application of oxytetracycline above, tetracycline resistance can be and gentamicin in plant caused by differences in cell agriculture and implications of permeability [20] and porin expression [21], or as a result of the their use outside clinical (over)expression of multi-drug resistance settings. pumps [22]. To conclude, tetracycline- resistant pathogens of clinical Although modest in relation to their importance rarely owe their limited application in human and veterinary susceptibility to mutations in their 16S medicine, streptomycin, oxytetracycline, rRNA genes [3]. and gentamicin are the most widely used antimicrobials in horticulture [28]. As to gentamicin, resistance to These substances are generally sprayed aminoglycosides is mainly due to to the aerial parts of crops or to covalent modification of the antibiotics farmlands at concentrations ranging by a diverse collection of enzymes [23]. from 50 to 300 parts per million [29]. In this context, O-phosphotransferases (APH) and O-adenyltransferases (ANT) Calcium complexes or hydrochloride employ ATP to alter hydroxyl functions. salts of oxytetracycline are N-acetyltransferases (AAC), on the other internationally sold under the trademarks hand, make use of acetyl-coenzyme A to Glomycin®, Terrafungine®, Riomitsin®, modify amino groups [10]. Structural Hydroxytetracycline®, Berkmycin®, modifications of gentamicin on positions Biostat®, Impercin®, Oxyatets®, 2” and 3’ by APH, 2” by ANT, and 2’, 3 Mycoshield®, and Agricultural and 6’ by AAC result in a severe terramycin® for control of fire blight of reduction of the ability of this drug to pears, pear decline, bacterial spot on bind to its