Impact of untreated tannery wastewater in the evolution of multidrug-resistant bacteria in Bangladesh

Abstract

The tannery industry produces one of the worst contaminants, and unsafe disposal in nearby waterbodies and landfills has become an imminent threat to public health, especially when the resulting multidrug-resistant bacteria and heavy metals enter community settings and animal food chains. In this study, we have collected 10 tannery wastewater (TWW) samples and 10 additional non-tannery wastewater (NTW) samples to compare the chemical oxygen demand (COD), pH, biological oxygen demand (BOD), dissolved oxygen (DO), total dissolved solids (TDS), chromium concentration, bacterial load, and antibiotic resistance profiles. While COD, pH, and chromium concentration data were previously published from our lab, this part of the study uncovers that TWW samples had a significantly higher bacterial load, compared to the non-tannery wastewater samples (5.89 × 10⁴ and 9.38 × 10³ cfu/mL, respectively), higher BOD and TDS values, and significantly lower DO values. The results showed that 53.4, 46.7, 40.0, and 40.0% of the TWW isolates were resistant to ceftriaxone, erythromycin, nalidixic acid, and azithromycin, respectively. On the other hand, 20.0, 30.0, 50.0, and 40.0% of the NTW isolates were resistant to the same antibiotics, respectively. These findings suggest that the TWW isolates were more resistant to antibiotics than the NTW isolates. Moreover, the TWW isolates exhibited higher multidrug resistance than the NTW isolates, 33.33, and 20.00%, respectively. Furthermore, spearman correlation analysis depicts that there is a negative correlation between BOD and bacterial load up to a certain level (r = - 0.7749, p = 0.0085). In addition, there is also a consistent negative correlation between COD and bacterial load (r = - 0.7112, p = 0.0252) and TDS and bacterial load (r = - 0.7621, p = 0.0104). These findings suggest that TWW could pose a significant risk to public health and the environment and highlight the importance of proper wastewater treatment in tannery industries.

Keywords: Bacterial load; Biological oxygen demand; Chemical oxygen demand; Chromium; Dissolved oxygen; Multidrug-resistance; Tannery wastewater; Total dissolved solids.

Fig. 1

Flowchart of the study.

Fig. 2

Comparison of physicochemical properties in TWW and NTW. The mean COD level (A), mean BOD (B), mean TDS (D), mean Cr level (E), and mean bacterial load (F) were substantially higher in TWW compared to non-tannery control wastewater with the exception to Mean DO values (C). NTW: non-tannery wastewater; TWW: tannery wastewater.

Fig. 3

Association of COD, BOD, TDS, DO, and chromium with bacterial load in TWW. (A) Bacterial load decreases as the COD level increases beyond a certain level. (B) BOD showed a slightly stronger negative correlation than that of COD. (C) TDS shows a strong negative correlation with bacterial load, suggesting that as the concentration of dissolved solids increases, the bacterial population tends to decrease. (D) Bacterial load is inversely proportional to the chromium concentration in TWW. (E) DO level shows a weak positive correlation with bacterial load.

Fig. 4

Antimicrobial resistance profile of TWW and NTW isolates. Amongst the TWW isolates, CN exhibited the highest antimicrobial activity, while CRO was found the least effective (A). CN showed the greatest effectiveness against NTW isolates while NA was the least (B). A comparison of resistance prevalence in TWW and NTW isolates reveals that the overall resistance to most antibiotics was higher in TWW isolates than of NTW isolates (C). Antibiotic susceptibility was determined against erythromycin (E), ampicillin (AMP), tetracycline (TET), ciprofloxacin (CIP), nalidixic acid (NA), ceftriaxone (CRO), chloramphenicol (C), gentamycin (CN), kanamycin (K), and azithromycin (AZM). TWW: Tannery wastewater, NTW: Non-tannery wastewater.

Fig. 5

Resistance pattern of TWW and NTW isolates against different antimicrobial classes. (A) Antibiotic resistance pattern of bacterial strains collected from TWW samples. Aminoglycosides and cephalosporins were the most and least sensitive antimicrobial classes against TWW isolates. (B) Antibiotic resistance pattern of bacterial strains collected from NTW samples. The NTW isolates were most resistant to quinolones. (C) Resistance pattern against one or multiple classes of antibiotics. The comparative analysis reveals that TWW isolates were constantly multiple drug-resistant compared to NTW.