Pigs, Fish & Antimicrobial Resistance
Integrated livestock-fish aquaculture commonly uses livestock manure (animal excreta, urine, and feed leftovers) as
pond fertilizers in Southeast Asia. This
process aids planktonic and microorganism growth that the fish are able to feed
on.
The animal
waste often contains antimicrobial residues and resistant bacteria, which are
subsequently released into fishponds; residues often originate from antibiotics
incorporated within the livestock feed. This may favor selection of antimicrobial-resistant
bacteria, with a potential risk of resistance genes being transmitted to a range
of aquatic bacteria.
Son Thi Thanh Dang and friends (2011) aimed to determine the association between the provision of pig feed, containing
antimicrobials, and the development of antimicrobial resistance. Researchers
measured resistance in E. coli and Enterococcus strains, isolated
from pig manure and water-sediment pond samples, from an experimental
pig-fish farm located in periurban Hanoi, Vietnam. Strains of Enterococcus spp. and Escherichia coli were chosen because they are often
used as indicators of fecal contamination and to monitor antimicrobial
resistance, since they are common in environments contaminated by human and
animal feces.
The antibiotics used were tetracycline
(TET) and enrofloxacin (ENR), these were incorporated as
part of the feed provided to nine piglets over a course of four
months (see below for feeding regime).
Month 1, feed without
antimicrobials
Month 2, feed with TET and
ENR
Month 3, feed without
antimicrobials
Month 4, feed with TET and
ENR
As well as testing resistance to ENR and
TET, resistance to nalidixic acid (NAL) was examined in E. coli, since
resistance to fluoroquinolones is believed to be a two-step process, in which
the first step is NAL resistance caused be a single mutation.
MIC values, (minimum inhibitory concentration) for NAL
and ENR showed that E. coli and
Enterococcus spp. displayed
significantly higher frequencies of resistance toward NAL and ENR during the
months when pigs were provided feed containing antimicrobials, in both manure
and water-sediment samples.
However, the study found TET resistance for both strains was high throughout
the study period, indicating that TET-resistant E. coli and Enterococcus
spp. were present in the piglets prior to the start of the experiment. PCR-identification
revealed related occurrences of several Enterococcus spp. in the
water-sediment and manure samples - suggesting that these species may have
originated from the pig manure.
In summary, this paper shows that E.
coli and Enterococcus spp. isolated
from manure and water-sediment samples developed resistance to NAL and ENR, but
not to TET, and this was connected with the provisioning of feed containing the
two antibiotics. However, the spread of antimicrobial resistance to other
aquatic bacteria in such integrated farms needs further assessment regarding
the possible human health and food safety.
Son Thi Thanh Dang, Andreas Petersen, Dung Van Truong, Huong Thi Thanh
Chu and Anders Dalsgaard4. (2011). Impact of Medicated Feed on the
Development of Antimicrobial Resistance in Bacteria at Integrated
Pig-Fish Farms in Vietnam. APPLIED AND ENVIRONMENTAL MICROBIOLOGY. 77 (11), 4494–4498.
Sean, Do the authors give any reasoning for this feeding regime?
ReplyDeleteI do not agree with the choice of ecoli as an "indicator" in this study as there is a consensus that ecoli is not an appropriate indicator organism. This is because they can come from a variety of sources like runoff as well as fecal pollution. They also so not reflect the survival of pathogens so they so do not meet one of the key criteria for a reliable indicator organism. Enterococcus sp on the the other hand are thought to be more reliable so it is interesting as to why the authors used them both when their reliability as indicators is in stark contrast. What do you think of this? do know why the authors have done this?
Hi James,
ReplyDeleteThanks for your comment! I had similar thoughts regarding the feeding regime too, how would you know if the antibiotics administered in month 2 affected the results in month 3? One would assume that a logical progression would be, no antibiotics for months 1 & 2, and incorporated antibiotics in months 3 & 4. they did leave 3 days between switching regimes though personally i don't know if this would be long enough ...
With regards to the indicators chosen, all the authors put forward is:
"ntero- coccus spp. and Escherichia coli are common in environments contaminated by human and animal feces and therefore are often used as indicators of fecal contamination and to monitor antimicrobial resistance."
However this form of integrated farm is a traditional method to the area and as far as I can tell used domestically. So they may have chosen them because they are prolific in that pond-farm environment and may be concerned with resistance being transferred to forms pathogenic to people. though this is purely inferred on my account.
Cheers. You have helped to shed some light on this. This study was done in Vietnam so maybe the use of e.coli would have been more applicable there.
ReplyDeleteAs the criteria (as far as I know) was decided by the US Environmental Protection Agency I can see how you may be right!
ReplyDelete