Antimicrobial resistance among Pseudomonas spp. and the Bacillus cereus group isolated from Danish agricultural soil

Pollution-induced community tolerance of soil microbial communities caused by the antibiotic sulfachloropyridazine

Little is known about the environmental hazards linked to the treatment of farm animals with antibiotics and subsequent spreading of manure, especially regarding soil microbial communities. In this investigation, pollution-induced community tolerance (PICT) of bacteria from soils artificially spiked with the sulfonamide sulfachloropyridazine (SCP) was investigated. Tolerance of the bacterial communities after 3 weeks' exposure to SCP was determined by analyzing the sensitivity of 31 microbial metabolic processes in microtiter plates. Bacterial suspensions extracted from soils containing higher concentrations of SCP showed an increased tolerance of their metabolic activities to this antibiotic. An increase in tolerance by 10% was found at 7.3 mg/kg dw SCP. The PICT effect could be demonstrated by both a shift in the tolerance of the average of all metabolic activities and a shift of the physiological process sensitivity distributions made up from the single metabolic processes. The PICT effect was accompanied by smaller changes in the community-level physiological profile (CLPP). To conclude, PICT has been shown to be a versatile and illustrative method for the detection of the effects of antibacterial agents on soil microorganisms.

Identification of a new ribosomal protection type of tetracycline resistance gene, tet(36), from swine manure pits

Previously, only one ribosome protection type of a tetracycline resistance gene, tetQ, had been identified in Bacteroides spp. During an investigation of anaerobic bacteria present in swine feces and manure storage pits, a tetracycline-resistant Bacteroides strain was isolated. Subsequent analysis showed that this new Bacteroides strain, Bacteroides sp. strain 139, did not contain tetQ but contained a previously unidentified tetracycline resistance gene. Sequence analysis showed that the tetracycline resistance gene from Bacteroides sp. strain 139 encoded a protein (designated Tet 36) that defines a new class of ribosome protection types of tetracycline resistance. Tet 36 has 60% amino acid identity over 640 aa to TetQ and between 31 and 49% amino acid identity to the nine other ribosome protection types of tetracycline resistance genes. The tet(36) region was not observed to transfer from Bacteroides sp. strain 139 to another Bacteroides sp. under laboratory conditions. Yet tet(36) was found in other genera of bacteria isolated from the same swine manure pits and from swine feces. Phylogenetic analysis of the tet(36)-containing isolates indicated that tet(36) was present not only in the Cytophaga-Flavobacter-Bacteroides group to which Bacteroides sp. strain 139 belongs but also in gram-positive genera and gram-negative proteobacteria, indicating that horizontal transfer of tet(36) is occurring between these divergent phylogenetic groups in the farm environment.

Arming the enemy: the evolution of resistance to self-proteins

A remarkable range of novel antibiotics is attracting increasing interest as a major new weapon in the campaign against bacterial infection. They are based on the toxic peptides that provide the innate immune system of animals, and it is claimed that bacteria will be unable to evolve resistance to them because they attack the 'Achilles' heel' of bacterial membrane structure. Both experimental evidence and theoretical arguments suggest that this claim is doubtful. If so, the introduction of these substances into general use may provoke the evolution of resistance to our own defence proteins and thus compromise our natural defences against infection.

Tylosin-resistant bacteria cultivated from agricultural soil

In this study we analyzed the numbers and types of cultivable tylosin-resistant bacteria from six agricultural soils that differed with respect to their association with subtherapeutic antibiotic use. The proportion of tylosin-resistant bacteria to the total number of bacteria cultivated was significantly higher (7.2-16.5%) at three sites affected by subtherapeutic antibiotic use compared to three sites unaffected by subtherapeutic antibiotic use (0.7-2.5%). We also detected differences in the types of cultivable tylosin-resistant bacteria. At a site affected by subtherapeutic antibiotic use, we detected tylosin-resistant bacteria from the alpha- and beta-subdivisions of Proteobacteria. In contrast, at a site unaffected by subtherapeutic use, we detected only Streptomyces-like (high-G+C Gram-positive) tylosin-resistant bacteria. Our results suggest a link between subtherapeutic use of antibiotics and the numbers and types of antibiotic-resistant bacteria in nearby soil. However, other factors, such as soil type and temporal variation, may have also contributed to the differences observed.

Presence of erm genes among macrolide-resistant Gram-positive bacteria isolated from Danish farm soil

The presence of erm genes was investigated among macrolide-resistant Gram-positive bacteria isolated from soil samples collected from four Danish farms that had been treated with animal waste. Soil samples were collected before, a few days after spread and 1 1/2 months and 5 months later. In 33% (9/27) of these isolates, an erm gene was detected using PCR. Eight isolates were positive for erm(B) and one isolate was positive for erm(C). No isolates contained erm(A), erm(D) or erm(F). The positive isolates were identified to genus level. Two erm(B) positive isolates were identified as Enterococcus spp., and the erm(C)-positive isolate as a Streptococcus spp., probably indicating survival from animal waste. The remaining six erm(B) positive isolates all belonged to the Bacillus cereus group. The erm(B) gene has not previously been identified in B. cereus but is frequently found among enterococci. This result could indicate horizontal transfer from bacteria of animal origin to indigenous soil bacteria.

The identification of a tetracycline resistance gene tet(M), on a Tn916-like transposon, in the Bacillus cereus group

In order to investigate whether resistance genes present in bacteria in manure could transfer to indigenous soil bacteria, resistant isolates belonging to the Bacillus cereus group (Bacillus cereus, Bacillus anthracis and Bacillus thuringiensis) were isolated from farm soil (72 isolates) and manure (12 isolates) samples. These isolates were screened for tetracycline resistance genes (tet(K), tet(L), tet(M), tet(O), tet(S) and tet(T)). Of 88 isolates examined, three (3.4%) isolates carried both tet(M) and tet(L) genes, while four (4.5%) isolates carried the tet(L) gene. Eighty-one (92.1%) isolates did not contain any of the tested genes. All tet(M) positive isolates carried transposon Tn916 and could transfer this mobile DNA element to other Gram-positive bacteria.