The total percentage of strains from farming seawater and
oysters with resistances to the 10 antibiotics are presented in
Fig. 1 . The data showed minor differences between strains from
farming seawater and oysters in their responses to the same
antibiotics. Correlation analysis indicated that a significant and
positive correlation existed between the seawater-derived strains
and the oyster-derived strains (r ¼ 0.929).
3.2. Variation in resistance rates of 1,050 strains to every antibiotic
in different months
The water temperatures ranged from 18.7 1C to 32.8 1C, with
the highest temperatures in June. Salinity ranged from 0.2% to
2.76%. The pH ranged from 7.3 to 8.39. DO ranged from 2.75 to
7.67 mg/L.
The resistance rates of bacterial strains isolated from seawater
and oysters at different months are presented in Figs. 2 and 3,
respectively. The rates fluctuated at regular intervals and
resembled zigzags on the graph. The peaks of the resistance rates
of seawater-derived strains to multiple antibiotics mostly overlapped in April, June, September, and November, whereas those of
oyster-derived strains overlapped in April, July, and September.
The resistant rate to F every month was obviously higher than the
resistance to all other tested antibiotics (Figs. 2 and 3). The
maximum resistance values to F were 93.3% in April for
seawater-derived strains and 97.1% in July for oyster-derived
strains, respectively. The resistance rates of seawater-derived
bacteria to C, ENX, and CIP were low (less than 20%). Furthermore,
the resistance rates of oyster-derived bacteria to C and ENX were
low. Many strains showed resistance to F (at resistance rates
higher than 40%), P, and RA. The total resistance rate to F, which
is a banned drug in aquaculture, was much higher than the
resistance rates of other tested antibiotics (Fig. 1 ). The maximum
resistance values were 71.8% for seawater-derived strains and
75.3% for oyster-derived strains, respectively.
The trends of multi-resistance rates for seawater- and oysterderived strains were similar (Fig. 4). In April and May, during the
early development stages of the oysters, the multidrug resistance
rate of oyster-derived strains were much lower than the multidrug
resistance rate of seawater-derived strains. However, after June,
the multidrug resistance rates of o