Synthesis and characterization of a tetracycline-platinum (II) complex active against resistant bacteria

A tetracycline-platinum(II) complex, [PtCl2(C22H24N2O8)], was synthesized and characterized by elemental analysis, conductivity and thermogravimetric analyses, and infrared spectroscopy. The interaction of tetracycline (Tc) with platinum(II) ions was also studied in aqueous solution by 1H NMR and circular dichroism spectroscopies. Tetracycline forms a 1:1 complex with platinum via the oxygen of the hydroxyl group at the A ring and that of the amide group. The complex is as efficient as tetracycline in inhibiting the growth of two Escherichia coli sensitive bacterial strains and six times more potent against E. coli HB101/pBR322, a bacterial strain resistant to tetracycline. This finding is very important because the use of tetracycline to treat bacterial infections has declined due to the emergence of resistant organisms.     

Specificity and mechanism of tetracycline resistance in a multiple drug resistant strain of Escherichia coli

Izaki, Kazuo (University of Tokyo, Tokyo, Japan), Kan Kiuchi, and Kei Arima. Specificity and mechanism of tetracycline resistance in a multiple drug resistant strain of Escherichia coli. J. Bacteriol. 91:628-633. 1966.-A decrease in the uptake of tetracycline occurred concurrently with a rise in the level of resistance of a multiple drug resistant strain of Escherichia coli grown in the presence of tetracycline. Although the strain was also resistant to streptomycin and chloramphenicol, growth in the presence of these two antibiotics did not influence the uptake of tetracycline. The induction of resistance, or decreased uptake of tetracycline, was dependent on growth of the organism in the presence of the drug. Decreased uptake of tetracycline could not be induced in a sensitive strain of the same organism under conditions suitable for induction of the resistant strain. The decrease in accumulating power of the resistant organism cultured in the presence of tetracycline does not appear to be due to selection of a resistant strain from cultures containing both resistant and sensitive strains.     

Makeup and properties of E. coli cells with a varying level of resistance to tetracycline]

Lipopolysaccharide composition of tetracycline sensitive and resistant strains of E. coli was studied comparatively. It was shown that that resistance of E. coli to tetracycline was probably due to the differences in the lipopolysaccharide component composition of the outer membrane. On the basis of the activity comparison of the Mg2+- and Ca2+-activated ATP-ase of the membrane fraction of the tetracycline sensitive and resistance strains of E. coli it was concluded that the resistance development in the strains tested to tetracycline was not associated with the changes in the ATP-ase activity.     

Multicopy Tn10 tet plasmids confer sensitivity to induction of tet gene expression.

We inserted the Tn10 tetracycline resistance determinant (tet) into the multicopy plasmid pACYC177, and we examined the phenotype of Escherichia coli K-12 strains harboring these plasmids. In agreement with others, we find that Tn10 tet exhibits a negative gene dosage effect. Strains carrying multicopy Tn10 tet plasmids are 4- to 12-fold less resistant to tetracycline than are strains with a single copy of Tn10 in the bacterial chromosome. In addition, we find that multicopy tet strains are 30- to 100-fold less resistant to the tetracycline derivative 5a,6-anhydrotetracycline than are single-copy tet strains. Multicopy tet strains are, in fact, 10- to 25-fold more sensitive to anhydrotetracycline than are strains that lack tet altogether. The hypersensitivity of multi-copy strains to anhydrotetracycline is correlated with the effectiveness of anhydrotetracycline as an inducer of tet gene expression, rather than its effectiveness as an inhibitor of protein synthesis. Anhydrotetracycline is 50- to 100-fold more effective than tetracycline as an inducer of tetracycline resistance and as an inducer of beta-galactosidase in strains that harbor tet-lac gene fusions. In contrast, anhydrotetracycline appears to be two- to fourfold less effective than tetracycline as an inhibitor of protein synthesis. Both anhydrotetracycline and tetracycline induce synthesis of tet polypeptides in minicells harboring multicopy tet plasmids. Differences between E. coli K-12 backgrounds influence the tetracycline and anhydrotetracycline sensitivity of multicopy strains; ZnCl2 enhances the tetracycline and anhydrotetracycline sensitivity of these strains two- to threefold. We propose that the overexpression of one or more Tn10 tet gene products inhibits the growth of multicopy tet strains and accounts for their relative sensitivity to inducers of tet gene expression.     

Comparison of therapeutic effects of antibiotics of the tetracycline group in the treatment of anthrax caused by a strain inheriting tet-gene of plasmid pBC16]

In vivo and in vitro efficacy of tetracyclines was studied with respect to anthracic infection induced by a tetracycline-resistant resistant strain containing plasmid pBC16. The plasmid-containing strain was resistant to tetracycline, doxycycline and minocycline, the MICs exceeding those for the initial strain 500, 640 and 80 times, respectively. There was no therapeutic effect of tetracycline and doxycycline in the treatment and urgent prophylaxis of anthracic infection caused by the tetracycline-resistant strain of Bacillus anthracis. High therapeutic efficacy of minocycline in the average therapeutic concentrations was shown irrespective of the contaminating doses and strains. Minocycline was recommended for treatment and urgent prophylaxis of anthracic infection caused by tetracycline-resistant B. anthracis strains.     

Ampicillin inactivation in the caecum of axenic, gnotoxenic and conventional lambs: interaction with resistant or sensitive Escherichia coli

The fate of orally administered ampicillin was studied in axenic lambs, in gnotoxenic lambs given a complex microflora and a mixture of ampicillin resistant and/or sensitive strains of Escherichia coli, and in conventional lambs. In axenic lambs or animals with a sensitive microflora, antibiotic concentrations of 500-1600 micrograms ml-1 were detected in the intestine, and most of the ampicillin passed through the small intestine and entered the large intestine, within 12-15 h of administration. These antibiotic concentrations were sufficient to decrease the numbers of ampicillin-sensitive E. coli from 10(8)-10(9) bacteria ml-1 to about 10(5)-10(6) bacteria ml-1 by 8 h after ampicillin administration. Second and third doses of antibiotic had no further effect on the bacterial count. Administration of ampicillin to animals hosting ampicillin-resistant E. coli resulted in a significant inactivation of the antibiotic in the intestine. As might be expected there was little reduction in the numbers of these organisms. These results are similar to those observed in conventional lambs hosting resistant E. coli as the dominant colibacillary flora.     

Antimicrobial susceptibilities of Campylobacter sp strains isolated from humans in 2005 to 2006 in Bielsko-Biala Region, Poland

Campylobacter jejuni and Campylobacter coli are frequent causes of bacterial gastroeneritis in humans worldwide. Campylobacteriois is usually a self-limiting disease and therapy with antibiotics is required in severe clinical infections. The objective [corrected] of this study was to determine the antibiotic resistance of C. jejuni and C. coli isolated from humans with diarrhea during 2005-2006 in Bielsko-Biala region in Poland. The MICs of ciprofloxacin, tetracycline, erythromycin, gentamicin and ampicillin were determined by the E-test method. It was observed that 23 % and 6% C. jejuni isolates were resistant to two and three antibiotics, respectively. All isolates of Campylobacter sp. were sensitive to erythromycin and gentamicin. From the 69 C. jejuni strains 58% were resistant to ciprofloxacin, 23% to tetracycline and 17% to ampicillin. From the 8 C. coli strains all were resistant to ciprofloxacin, 62,5% to ampicillin and 12,5% to tetracycline.     

Frequency of isolation and susceptibility to antibiotics of Escherichia coli strains isolated from blood

The aim of our study was the analysis of Escherichia coli strains obtained from patients of University Hospital No 1 of dr A. Jurasz Collegium Medicum of L. Rydygier in Bydgoszcz Nicolaus Copernicus University in Toruń and State Infectious Diseases Observatory Hospital of T. Browicz in Bydgoszcz, between 2007 and 2010. Among all microorganisms isolated from blood was 8.7% E. coli strains and 45.1% of all rods from Enterobacteriaceae family. Number of E. coli isolations from positive blood samples was: 64 in 2007, 69 in 2008, 77 in 2009 and 26 in the first half of 2010 year. The highest percentage of E. coli strains were obtained from patients of the Transplantology and Surgery Clinic (16.1%), the Nephrology and Internal Diseases Clinic with the Dialysis Centre (14.0%), the Pediatric, Hematology and Oncology Clinic (13.6%) and the Anesthesiology and Intensive Care Clinic (13.6%). All analysed strains were susceptible to carbapenems, amikacin and tygecycline. The highest percentage of resistant strains were observed to ampicillin (70.7%), piperacillin (43.9%), tetracycline (42.8%) and doxycycline (38.8%). During four years of study 4 (6.3%), one, three and two E. coli strains with ESBL were isolated, respectively.     

In vivo formation of transmissible resistance factor by recombination between nontransmissible resistance factor and Col B factor.

Germ-free swine were artificially contaminated with tetracycline (TC) sensitive strains of Escherichia coli and Klebsiella pneumoniae. One of these strains, E. coli 3306, was infected with a plasmid carrying kanamycin (KM) resistance, i.e., T-kan factor. Another strain, E. coli P-5, carried a conjugally transferable Col B factor. Among the nine strains used, only E. coli P-38 became TC-resistant after TC administration. Three types of TC-resistance E. coli P-38 strains were found; (a) one strain carried nontransferable TC resistance and could not produce colicin, (b) one strain carried TC resistance with a high transmission frequency which could not produce colicin, and (c) one strain carried TC resistance with a low transmission frequency that could produce colicin B. Genetic studies disclosed that the transmissible TC resistance factors, i.e., Rms105 (group b) and Rms104 (group c), were formed by recombination between Col B factor and nontransmissible TC-resistance (tet) determinant which appeared in E. coli P-38 mutants.     

Antibiotic resistance among coliforms and Pseudomonas spp. from bodies of water around Port Harcourt, Nigeria

Samples from municipal waste water, the Bonny River estuary and wells in and around Port Harcourt were examined for bacteriological quality over a 9 month period. A total of 157 Pseudomonas spp., 133 Escherichia coli and 282 other coliforms were isolated and tested for the incidence of resistance to 10 antibiotics. All of the Pseudomonas spp. were resistant to at least one of the antibiotics while 96.2% were resistant to two or more. Most (83.5%) of the E. coli and other coliforms (91.8%) were resistant to at least one antibiotic. All strains were susceptible to gentamicin. Minimal inhibitory concentrations of ampicillin and tetracycline for E. coli ranged from 6.25 to 50 and 6.25 to 12.5 micrograms/ml, respectively. Minimal inhibitory concentrations of ampicillin and tetracycline were 1000 and 25 micrograms/ml for the Pseudomonas strains. The high incidence of bacterial resistance to antibiotics is discussed in relation to the widespread use of antibiotics, and possible public health implications.     

Interactions of the anticancer antibiotic altromycin B with copper(II), palladium(II) and platinum(II) ions and in vitro activity of the formed complexes

Interaction of the anticancer antibiotic altromycin B with Cu(II), Pd(II) and Pt(II) ions was studied using 1H-NMR, EPR, electronic absorption and circular dichroism spectroscopy. The results derived from NMR studies where that the Pt(II) and Pd(II) ions interact with the nitrogen atom of the dimethylamino group of the C(10)-disaccharide, while the C(2)-epoxide group does not participate and remains intact. Cu(II) ions interact in a different way with altromycin B as was concluded by EPR and circular dichroism spectra. Altromycin B coordinates to the Cu(II) ions via the oxygen atoms of the C(11) phenolic and the C(12) carbonyl group while the nitrogen atom does not participate in the complexation. The presence of these metal ions improves the stability of altromycin B in solution. These complexes were studied in vitro against K562 leukemia sensitive and doxorubicin-resistant cells and GLC4 lung tumor cells, sensitive and doxorubicin-resistant. The activity of the complexes compared to the free drug is improved against resistant cells and is affected moderately against sensitive cells. Finally, 20% of platinum added as altromycin B metal complex entered GLC4 cells.     

Transfer of multiple drug resistance plasmids between bacteria of diverse origins in natural microenvironments.

Plasmids harboring multiple antimicrobial-resistance determinants (R plasmids) were transferred in simulated natural microenvironments from various bacterial pathogens of human, animal, or fish origin to susceptible strains isolated from a different ecological niche. R plasmids in a strain of the human pathogen Vibrio cholerae O1 E1 Tor and a bovine Escherichia coli strain were conjugated to a susceptible strain of the fish pathogenic bacterium Aeromonas salmonicida subsp. salmonicida in marine water. Conjugations of R plasmids between a resistant bovine pathogenic E. coli strain and a susceptible E. coli strain of human origin were performed on a hand towel contaminated with milk from a cow with mastitis. A similar conjugation event between a resistant porcine pathogenic E. coli strain of human origin was studied in minced meat on a cutting board. Conjugation of R plasmids between a resistant strain of the fish pathogenic bacterium A. salmonicida subsp. salmonicida and a susceptible E. coli strain of human origin was performed in raw salmon on a cutting board. R plasmids in a strain of A. salmonicida subsp. salmonicida and a human pathogenic E. coli strain were conjugated to a susceptible porcine E. coli strain in porcine feces. Transfer of the different R plasmids was confirmed by plasmid profile analyses and determination of the resistance pattern of the transconjugants. The different R plasmids were transferred equally well under simulated natural conditions and under controlled laboratory conditions, with median conjugation frequencies ranging from 3 x 10(-6) to 8 x 10(-3). The present study demonstrates that conjugation and transfer of R plasmids is a phenomenon that belongs to the environment and can occur between bacterial strains of human, animal, and fish origins that are unrelated either evolutionarily or ecologically even in the absence of antibiotics. Consequently, the contamination of the environment with bacterial pathogens resistant to antimicrobial agents is a real threat not only as a source of disease but also as a source from which R plasmids can easily spread to other pathogens of diverse origins.     

Transfer of multiple drug resistance plasmids between bacteria of diverse origins in natural microenvironments

Plasmids harboring multiple antimicrobial-resistance determinants (R plasmids) were transferred in simulated natural microenvironments from various bacterial pathogens of human, animal, or fish origin to susceptible strains isolated from a different ecological niche. R plasmids in a strain of the human pathogen Vibrio cholerae O1 E1 Tor and a bovine Escherichia coli strain were conjugated to a susceptible strain of the fish pathogenic bacterium Aeromonas salmonicida subsp. salmonicida in marine water. Conjugations of R plasmids between a resistant bovine pathogenic E. coli strain and a susceptible E. coli strain of human origin were performed on a hand towel contaminated with milk from a cow with mastitis. A similar conjugation event between a resistant porcine pathogenic E. coli strain of human origin was studied in minced meat on a cutting board. Conjugation of R plasmids between a resistant strain of the fish pathogenic bacterium A. salmonicida subsp. salmonicida and a susceptible E. coli strain of human origin was performed in raw salmon on a cutting board. R plasmids in a strain of A. salmonicida subsp. salmonicida and a human pathogenic E. coli strain were conjugated to a susceptible porcine E. coli strain in porcine feces. Transfer of the different R plasmids was confirmed by plasmid profile analyses and determination of the resistance pattern of the transconjugants. The different R plasmids were transferred equally well under simulated natural conditions and under controlled laboratory conditions, with median conjugation frequencies ranging from 3 x 10(-6) to 8 x 10(-3). The present study demonstrates that conjugation and transfer of R plasmids is a phenomenon that belongs to the environment and can occur between bacterial strains of human, animal, and fish origins that are unrelated either evolutionarily or ecologically even in the absence of antibiotics. Consequently, the contamination of the environment with bacterial pathogens resistant to antimicrobial agents is a real threat not only as a source of disease but also as a source from which R plasmids can easily spread to other pathogens of diverse origins.     

Genetics of Sensitivity of Salmonella Species to Colicin M and Bacteriophages T5, T1, and ES18

Nearly all of 62 strains of Salmonella paratyphi B were sensitive to colicin M and phage T5 but resistant to phages T1 and ES18 and to colicin B. All tested S. typhimurium strains were resistant to colicin M and phage T5, and many were sensitive to phage ES18. A rough S. typhimurium LT2 strain given the tonA region of Escherichia coli or S. paratyphi B became sensitive to colicin M and phage T5. We infer that the tonA allele of S. paratyphi B, like that of E. coli, determines an outer membrane protein that adsorbs T5 and colicin M but not phage ES18, whereas the S. typhimurium allele determines a protein able to adsorb only ES18. The partial T1 sensitivity of a rough LT2 strain with a tonA allele from E. coli or S. paratyphi B and also the tonB(+) phentotype of an E. coli B trp-tonB Delta mutant carrying an F' trp of LT2 origin showed that S. typhimurium LT2 has a tonB allele like that of E. coli with respect to determination of sensitivity to colicins and phage T1. Rough S. paratyphi B, although T5 sensitive, remained resistant to T1 even when given F' tonB(+) of E. coli origin. Classes of Salmonella mutants selected as resistant to colicin M included: T5-resistant mutants, probably tonA(-); mutants unchanged except for M resistance, perhaps tolerant; and Exb(+) mutants, producing a colicin inhibitor (presumably enterochelin). Some Exb(+) mutants were resistant to a bacteriocin inactive on E. coli but active on all tested S. paratyphi B and S. typhimurium strains (and on nearly all other tested Salmonella). A survey showed sensitivity to colicin M in several other species of Salmonella.     

Mixed continuous culture experiments with an antibiotic-producing streptomycete andEscherichia coli

Four strains ofStreptomyces aureofaciens capable of producing different amounts of tetracycline have been grown in continuous culture with either of two strains ofEscherichia coli orBacillus pumillus. Each of the bacteria had faster specific growth rates than any of the streptomycetes.E. coli NTCT 5993 had a higher affinity for sucrose than didS. aureofaciens SR 11. In mixed culture experiments, the bacteria displaced the streptomycete under the following conditions: (a) when the streptomycete produced no tetracycline; (b) in tetracycline-producing cultures after a fraction of the bacteria had died—the bacterial population that subsequently developed was resistant to the drug; and (c) when using tetracycline-resistant bacteria. Under nutrient conditions leading to high antibiotic levels, it was possible to kill all the bacteria, and the streptomycete survived after a transient fall in mycelium and tetracycline levels. No stable mixed population was ever seen. Once tetracycline-resistant bacteria had displaced the streptomycete, and tetracycline concentrations had fallen, a sensitive bacterial population reappeared. Competition between sensitive and resistantE. coli NCTC 5993 in the chemostat confirmed the selective advantage of the sensitive strain. Results were discussed in terms of the role of antibiotics in nature.     

Isolation of nonsense suppressor mutants in Pseudomonas.

A strain of Escherichia coli harboring the drug resistance plasmid RP1 was treated with the mutagen N-methyl-N-nitro-N-nitro-N-nitrosoguanidine, and mutants were isolated in which ampicillin resistance had been lost due to an amber mutation in the plasmid. One of these mutants was again treated, and a strain was isolated in which tetracycline resistance was also lost due to an amber mutation in the plasmid. The plasmid containing amber mutations in the genes amp and tet was named pLM2. This plasmid could be transferred to strains of Pseudomonas aeruginosa, P. phaseolicola, and P. pseudoalcaligenes. Mutants resistant to ampicillin and tetracycline could not be obtained from P. phaseolicola carrying pLM2. However, strains of E. coli, P. aeruginosa, and P. pseudoalcaligenes carrying the plasmid did produce mutants simultaneously resistant to both antibiotics. All of the mutants of E. coli had developed nonsense suppressors since they became phenotypically lac+, although harboring a lac amber mutation, and formed plaques with amber mutants of phages PRR1 and PRD1 that attack organisms carrying RP1. Approximately 20% of the resistant mutants of P. aeruginosa and P. pseudoalcaligenes were sensitive to the amber mutant of PRD1. These mutants were of variable stability and grew somewhat more slowly than their parent strains. One of the suppressor mutants of P. pseudoalcaligenes, designated ERA(pLM2)S4, was used for the isolation of nonsense mutants of bacteriophage PHA6, a virus having a segmented genome of double-stranded ribonucleic acid and an envelope of lipids and proteins.     

Antibiotic resistance among coliforms and Pseudomonas spp. from bodies of water around Port Harcourt, Nigeria

Samples from municipal waste water, the Bonny River estuary and wells in and around Port Harcourt were examined for bacteriological quality over a 9 month period. A total of 157 Pseudomonas spp., 133 Escherichia coli and 282 other coliforms were isolated and tested for the incidence of resistance to 10 antibiotics. All of the Pseudomonas spp. were resistant to at least one of the antibiotics while 96.2% were resistant to two or more. Most (83.5%) of the E. coli and other coliforms (91.8%) were resistant to at least one antibiotic. All strains were susceptible to gentamicin. Minimal inhibitory concentrations of ampicillin and tetracycline for E. coli ranged from 6.25 to 50 and 6.25 to 12.5 μg/ml, respectively. Minimal inhibitory concentrations of ampicillin and tetracycline were 1000 and 25 μg/ml for the Pseudomonas strains. The high incidence of bacterial resistance to antibiotics is discussed in relation to the widespread use of antibiotics, and possible public health implications.     

Generation of novel plasmids in Escherichia coli S17-1(pSUP106)

When the highly metal-resistant acidophilic heterotrophic strain, Acidiphilium symbioticum KM2, was incubated with two Escherichia coli strains, viz. S17-1 (pSUP106) and K12, on a medium that supported growth of these two divergent species of different habitats, E. coli transconjugants were isolated that contained novel plasmids and were resistant to Zn(2+) (48 m M), Cu(2+) (12 m M), Ni(2+) (12 m M), chloramphenicol (50 microg/ml), and tetracycline (25 microg/ml). The transconjugant plasmids did not hybridize with any of the A. symbioticum KM2 plasmids. After curing of the plasmids, the transconjugants became sensitive to 12 m M Zn(2+), 12 m M Cu(2+), and 12 m M Ni(2+), but remained chloramphenicol and tetracycline resistant-the phenotypic markers that were originally present in pSUP106. That a part of pSUP106 was integrated into the chromosome of the transconjugants was evident from the hybridization of pSUP106 with chromosomal DNA of the cured derivatives of the transconjugants. Further, the transconjugant plasmids hybridized only with the chromosomal DNA of E. coli S17-1 and not with the chromosomal DNA of A. symbioticum KM2 or E. coli K12, suggesting their host chromosomal origin. Thus, the present study describes a unique event of genetic rearrangements in the E. coli strain S17-1 (pSUP106), resulting in the formation of novel plasmids conferring metal-resistance phenotypes in the cell.     

Pathogenicity island markers, virulence determinants malX and usp, and the capacity of Escherichia coli to persist in infants' commensal microbiotas

Virulence-associated genes in bacteria are often located on chromosomal regions, termed pathogenicity islands (PAIs). Several PAIs are found in Escherichia coli strains that cause extraintestinal infections, but their role in commensal bowel colonization is unknown. Resident strains are enriched in adhesins (P fimbriae and type 1 fimbriae), capsular antigens (K1 and K5), hemolysin, and aerobactin and mostly belong to phylogenetic group B2. Here, we investigated whether six pathogenicity islands and the virulence determinants malX and usp are associated with fitness of E. coli in the infant bowel microbiota. E. coli strains isolated from stools of 130 Swedish infants during the first year of life were examined for their carriage of PAI markers, malX, and usp by PCR. Carriage was related to strain persistence: long-term colonizers (≥12 months) carried significantly more of PAI II from strain CFT703 (II(CFT703)), IV(536,) and II(J96) and malX and usp than intermediate colonizers (1 to 11 months) and transient strains (<3 weeks). The accumulation of PAI markers in each individual strain correlated positively with its time of persistence in the colon. Phylogenetic group B2 accounted for 69% of long-term colonizers, 46% of intermediate colonizers and 14% of transient strains. These results support the hypothesis that some bacterial traits contributing to extraintestinal infections have in fact evolved primarily because they increase the fitness of E. coli in its natural niche, the colon; accordingly, they may be regarded as fitness islands in the gut.     

Characterization of a novel tetracycline resistance that functions only in aerobically grown Escherichia coli.

A tetracycline resistance (Tcr) gene that was found originally on two Bacteroides plasmids (pBF4 and pCP1) confers tetracycline resistance on Escherichia coli, but only when it is grown aerobically. Using maxicells, we have identified a 44-kilodalton protein which is encoded by the region that carries the Tcr gene and which may be the Tcr gene product. Localization experiments indicate that this 44-kilodalton protein is cytoplasmic. To determine whether the tetracycline resistance gene is expressed under anaerobic conditions, we have constructed a protein fusion between the Tcr gene and lacZ. In strains of E. coli carrying the fusion, beta-galactosidase activity was the same when the cells were grown under anaerobic conditions as when the cells were grown under aerobic conditions. This indicates that the tetracycline resistance gene product is made under anaerobic conditions but does not work. The failure of the Tcr protein to function under anaerobic conditions was not due to a requirement for function of the anaerobic electron transport system, because neither nitrate nor fumarate added to anaerobic media restored tetracycline resistance. Inhibition of the aerobic electron transport system with potassium cyanide did not prevent growth on tetracycline of cells containing the Tcr gene. A heme-deficient mutant, E. coli SHSP19, which carries the Tcr gene, was still resistant to tetracycline even when grown in heme-free medium. These results indicate that functioning of the Tcr gene product is not dependent on the aerobic electron transport system. Thus the requirement for aerobic conditions appears to reflect a requirement for oxygen. Spent medium from an E. coli strain carrying the Tcr gene, which was grown in medium containing tetracycline (50 micrograms/ml), did not inhibit growth of a tetracycline-susceptible strain of E. coli. Thus, the Tcr gene product may be detoxifying tetracycline.