Adaptive Responses and Molecular Characterization of Chromium-Resistant Bacteria from Industrially Polluted Environments of Bangladesh
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Date
2025-12-15
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Abstract
Background: Industrial effluents released from tannery, dyeing and battery industry contribute
largely to heavy metal pollution in both water and soil environments of Bangladesh. Chromium is
one of the most problematic metals as it remains in the environment for long time and shows high
toxicity, and its mobility increases under acidic conditions, making it risky for ecosystem and
human health. Microorganisms subjected to such conditions frequently evolve adaptation methods
that facilitate survival under metal stress, rendering them viable instruments for bioremediation.
The goal of this study was to find bacteria that can live in or break down heavy metals and then
make them more tolerant by passing them through higher and higher levels of heavy metals over
and over again.
Methods: Heavy metal degrading and tolerant bacteria were isolated from both water and soil
samples collected from different industrial polluted areas of Savar, Dhaka. At first, the bacterial
species were identified through selective media culture and common biochemical tests, and finally
PCR analysis with species specific primers was used to confirm the identification. Potassium
dichromate (K2Cr2O7) was used to test tolerance to chromium, the most prevalent heavy metal, in
broth dilution and then plate counting. Adaptive evolution was prompted by successive exposure
to progressively elevated chromium concentrations.
Stress-driven morphological changes were examined by Gram staining, microscopy, and colony
morphology was assessed visually on MacConkey agar.
Results: Water samples showed a wide pH variation (1.01–7.50), with several sites being highly
acidic and non-compliant with environmental standards. A total of 32 bacterial isolates were
recovered, with Pseudomonas, Klebsiella, Escherichia coli, Aeromonas, and Acinetobacter as
dominant genera. PCR confirmed most of the biochemically identified isolates. Among them, the
survival of E. coli and Klebsiella spp. in chromium salt was recorded as high as 1.5 mM. After
successive adaptation with increasing Chromium concentration, the MIC increased to 1.75 mM
and 1.5 mM for E. coli and Klebsiella, respectively. Colony morphology changed significantly in
Klebsiella under chromium stress, while E. coli showed no visible colony-level change. Adapted
cells exhibited reduced cell size, irregular morphology, altered cellular arrangement, whitish-
centered colony on the MacConkey plate22