The research team began with a market survey to identify which brands dominated local distribution points, then purchased samples in their original sealed condition in February 2026, transporting them in ice-packed coolers to the laboratory for analysis within 24 hours. This careful handling matters, because the goal was to capture the water exactly as consumers receive it. The eight brands selected represented the products most frequently encountered among vendors, retail shops, and street hawkers, making them a reasonable proxy for what residents of these university town and semi-urban communities actually drink every day. The authors acknowledge a limitation: with logistical constraints limiting the study to eight brands, the results may not capture the full diversity of sachet water on the market.

On the chemical side, the news was mostly reassuring. The pH of the samples ranged from 5.80 to 7.37, spanning mildly acidic to near-neutral conditions, with two brands falling squarely within recommended drinking water ranges. Electrical conductivity varied between 0.20 and 1.20 mS/cm, and total dissolved solids ranged from 100 to 600 parts per million, values consistent with treated or filtered water that has passed through purification steps such as reverse osmosis or demineralization. Turbidity, a measure of suspended particles, stayed low overall, peaking at 1.90 NTU in one sample. Dissolved oxygen, which healthy water bodies and potable supplies typically maintain at robust levels, ranged from 6.10 to 7.20 milligrams per liter across the samples.

Organic pollution indicators told a more nuanced story. Biochemical oxygen demand, chemical oxygen demand, and total oxygen demand, three complementary measures of how much oxygen is consumed by microbes and chemical reactions breaking down organic matter, all reached their highest values in the same sample: 1.80, 8.50, and 11.50 milligrams per liter respectively. That clustering suggests localized contamination or a higher load of biodegradable material in that particular brand. The statistical analysis reinforced this picture, revealing near-perfect positive correlations among the three oxygen demand parameters, and strong negative correlations between dissolved oxygen and both organic load and certain metals, a pattern consistent with microbial respiration depleting oxygen as organic matter decomposes.

The bacteriological results were where the study’s real alarm bells rang. Total heterotrophic bacterial counts, a broad measure of living bacteria in the water, ranged from zero to 110 colony-forming units per milliliter, with one brand exceeding the recommended limit of 100 CFU/ml. Coliform counts, which signal possible fecal contamination, peaked at 6.00 CFU/ml in one sample. The isolates recovered from the water produced the characteristic metallic green sheen on eosin methylene blue agar, appeared as Gram-negative rods under the microscope, and showed the classic biochemical fingerprint of E. coli: positive reactions for indole, methyl red, catalase, and motility, and negative reactions for Voges-Proskauer, citrate utilization, urease, and oxidase tests. Because E. coli lives in the intestinal tracts of humans and warm-blooded animals, its presence in drinking water is a universally recognized indicator of fecal contamination and sanitary lapses somewhere in the production chain, from untreated source water to unhygienic filling equipment or improper storage.

What happened next in the laboratory is what elevates this study from a routine water quality check to a public health warning. Using the Kirby-Bauer disk diffusion method on Mueller-Hinton agar, the researchers exposed the nine E. coli isolates to a panel of eight commercial antibiotics. The good news was limited but real: every isolate remained highly susceptible to ofloxacin, with inhibition zones of 26 to 34 millimeters, and cefoperazone performed well against most isolates. Nitrofurantoin showed selective activity, completely inhibiting some isolates while leaving others untouched. But the resistance side of the ledger was far heavier. Every isolate resisted augmentin, ampicillin, ceftazidime, cefuroxime, and gentamicin, with only slight inhibition against gentamicin in three isolates.

To quantify how alarming this pattern is, the team calculated the Multiple Antibiotic Resistance index, a ratio comparing the number of antibiotics an isolate resists to the number tested. Values above 0.2 are conventionally interpreted as evidence that bacteria originate from environments where antibiotics are frequently present. Every single isolate blew past that threshold, with MAR indices ranging from 0.63 to 0.75, meaning the bacteria shrugged off between 62.5 and 75 percent of the drugs thrown at them. Five isolates resisted six of the eight antibiotics. Such values point to high-risk source environments, the authors note, such as sewage systems, hospital effluents, animal waste runoff, or polluted surface waters, where continuous antibiotic pressure selects for resistant strains. The concern is not only that infections caused by these bacteria would be difficult to treat, but that E. coli serves as a reservoir of resistance genes transferable to other pathogens through horizontal gene transfer, a mechanism the World Health Organization has flagged as central to the global antimicrobial resistance crisis.

Heavy metals, by contrast, were largely a non-issue in these samples. Using atomic absorption spectrophotometry after acid digestion, the researchers measured copper, zinc, lead, nickel, cobalt, iron, and cadmium, finding all concentrations below permissible limits. Lead reached at most 0.006 parts per million and iron at most 0.160 parts per million. Three risk indices confirmed the picture: contamination factors for every metal at every site stayed below 1, the heavy metal pollution index ranged from roughly 1.72 to 2.84, far beneath the critical threshold of 100, and the health risk index, which models non-carcinogenic risk to an adult drinking two liters daily over a lifetime, ranged from 0.159 to 0.220, well below the threshold of 1.0 where concern begins. Interestingly, lead contributed the largest share of the modeled risk despite its low concentration, a reminder of how toxic the metal is even in trace amounts, and of why its very low regulatory reference dose gives it outsized weight in risk calculations.

The study’s central paradox is its most important takeaway. Chemically, the water passed. Microbiologically, it did not, at least not consistently. The authors conclude that while the physicochemical and heavy metal quality of the examined sachet water was generally acceptable, bacterial contamination and multidrug-resistant E. coli pose significant public health concerns. They call for strengthened regulatory monitoring, improved hygienic production practices, routine microbiological screening, proper storage systems, and antimicrobial resistance surveillance. They also outline the road ahead for their own work: future studies will employ membrane filtration, API 20E strips, and molecular identification techniques, with larger sample sizes, repeated seasonal sampling, and a broader range of brands to strengthen the risk assessment. For the residents of Ago-Iwoye, Oru, and the countless communities like them across Nigeria and beyond, the message is uncomfortable but clear: the water that looks clean and costs little may be delivering more than hydration, and the invisible cargo of resistance genes it may carry represents a slow-moving threat that chemical testing alone will never catch.

Subject of Research: Multidrug-resistant Escherichia coli contamination and public health risk assessment of sachet drinking water in southwestern Nigeria

Article Title: Multidrug-resistant Escherichia coli in selected sachet water and associated public health risk assessment in Ago-Iwoye and Oru, Ogun State, Nigeria

Article References: Samson, O. J., Adeyemi, J. O., Okunlola, D. O., Obadina, A. O., Omolade, O. A., Osikoya, O. V., Oliwo, A. A., Adeyemi, O. A., Ogunmoye, A. O., & Onajobi, I. B. (2026). Multidrug-resistant Escherichia coli in selected sachet water and associated public health risk assessment in Ago-Iwoye and Oru, Ogun State, Nigeria. Discover Chemistry, 3(1), Article 523. https://doi.org/10.1007/s44371-026-00975-0

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00975-0

Keywords: Escherichia coli, sachet water, antimicrobial resistance, drinking water quality, Nigeria, water microbiology, heavy metals, public health, MAR index, coliform contamination, Ogun State, risk assessment

Raw milk accounted for nearly all of the contamination. Nine of the 170 raw milk samples drawn from dairy cooperatives and collection centres tested positive, a prevalence of 5.29 percent, while a single positive emerged from the 60 samples taken from milking apparatus and transit containers. Strikingly, none of the 90 pasteurised milk samples and none of the 50 worker-related swabs harboured the organism. Eight of the ten raw milk collection centres surveyed showed at least some contamination, with the highest rates reaching 10 percent of samples at individual cooperatives. The pattern is unambiguous: the early, unprocessed stages of the milk chain are where L. monocytogenes persists, and the industrial treatments applied downstream are doing their job.

The complete absence of the pathogen from pasteurised milk is a direct endorsement of the high-temperature, short-time protocol used by the commercial dairies in the region, which heat milk to between 72 and 75 degrees Celsius for 15 seconds before packaging. L. monocytogenes is a non-spore-forming bacterium and is highly vulnerable to these time-temperature combinations, provided that strict hygiene is maintained after treatment to prevent recontamination. The finding matters because listeriosis, the disease the organism causes, carries hospitalisation and mortality rates that are exceptionally high among infants, the elderly, pregnant women, and immunocompromised patients. Unlike many foodborne infections that produce unpleasant but self-limiting illness, listeriosis can invade the bloodstream and central nervous system, making every point of control in the dairy chain a matter of clinical consequence.

What elevates the study beyond a routine prevalence survey is the molecular characterisation of the recovered isolates. Using polymerase chain reaction, the team screened the bacteria for five major virulence genes that govern the organism’s ability to invade and survive inside human cells. Every one of the ten isolates carried prfA, the master transcriptional regulator that switches on Listeria Pathogenicity Island 1, along with inlA, an internalin essential for breaching the intestinal barrier, and iap, a marker associated with invasion and virulence. The gene inlB, which works with inlA to drive uptake into host cells, was present in 90 percent of isolates, and hlyA, which encodes listeriolysin O, the pore-forming toxin that lets the bacterium escape the phagosome and spread cell to cell, was found in 90 percent as well. In practical terms, the strains circulating in Yazd’s raw milk carry the full genetic toolkit for invasive human disease.

One isolate lacked hlyA entirely, a detail with real diagnostic implications. Strains missing the gene for listeriolysin O are considered atypical and often non-haemolytic, meaning they fail the blood-agar tests that many laboratories rely on for routine identification. Such organisms can slip through phenotype-based screening undetected, yet they may possess alternative invasion mechanisms or acquire virulence factors through horizontal gene transfer. Their presence in the food supply is a recognised challenge for public health monitoring, because the diagnostic net designed to catch L. monocytogenes has holes exactly where these variants swim.

The antimicrobial resistance picture is more nuanced, and in some respects reassuring. The isolates showed the expected intrinsic resistance to nalidixic acid, penicillin G, ceftazidime, ceftriaxone, and cefepime, a pattern long documented in this species and the reason cephalosporins are never used to treat listeriosis. More encouragingly, all ten strains were fully susceptible to gentamicin, amikacin, levofloxacin, ciprofloxacin, vancomycin, chloramphenicol, trimethoprim-sulfamethoxazole, ampicillin-sulbactam, and erythromycin, the workhorses of clinical therapy. Resistance to tetracycline appeared in 30 percent of isolates, and imipenem showed a split profile, with half the isolates resistant and half intermediate. Twenty percent of isolates exceeded the multiple antibiotic resistance index threshold of 0.2, a value interpreted as a signature of environments with heavy antibiotic exposure, and the highest indices belonged to strains resistant to imipenem, clindamycin, and tetracycline simultaneously.

The genotyping of resistance determinants uncovered a finding that may prove the most strategically important in the entire study: several resistance genes were present in isolates that remained phenotypically susceptible. The lincosamide resistance genes lnuA and lnuB were detected in 90 and 80 percent of isolates respectively, the macrolide-lincosamide-streptogramin genes ermA and ermB in 40 and 30 percent, the aminoglycoside gene aadB in 30 percent, and the tetracycline genes tetA and tetM, which mediate efflux and ribosomal protection, in 30 percent each. Yet clindamycin susceptibility remained at 90 percent and erythromycin at 100 percent. This silent carriage of resistance genes, likely due to non-functional promoters, regulatory mutations, or the absence of environmental triggers needed for expression under laboratory conditions, means these strains hold the genetic blueprint to flip to a resistant phenotype through gene activation or horizontal transfer. Phenotypic testing alone would miss the threat entirely, which is why the authors argue that molecular surveillance must run alongside conventional susceptibility testing.

The coexistence of a complete virulence gene complement with a broad array of resistance determinants in the same small set of isolates paints a picture of strains that are both capable of causing severe disease and equipped to survive therapeutic pressure. The researchers contextualise their findings against international data: prevalence figures of 18.46 percent in South African milk samples, 28.26 percent in milk from aborted cows in Iraq, and 2.48 percent in bovine milk in Southwest China all confirm that milk is a substantial and recurring reservoir for this pathogen worldwide. Comparable genomic surveys in dairy products across India and China have likewise documented the ubiquitous presence of prfA, inlA, inlB, iap, and hlyA alongside multidrug resistance, suggesting that the Yazd findings reflect a global pattern rather than a local anomaly.

The study is not without limits, and the authors acknowledge them candidly. The low isolation rate yielded only ten confirmed strains, a sample too small to generalise precise resistance percentages across the entire province. Nevertheless, the detailed molecular characterisation provides a baseline for future large-scale monitoring in a region where such data have been scarce. The contamination of 80 percent of raw milk collection points points to hygiene failures at the earliest stage of the chain, where faecal contamination during milking, mastitic glands, and environmental reservoirs such as silage, soil, and water all offer the bacterium entry routes. The single positive from milking equipment reinforces the well-established capacity of L. monocytogenes to form resilient biofilms on food-contact surfaces, which complicates sanitation in processing facilities.

The practical message for consumers and regulators alike is clear. Pasteurisation works, and the zero detection rate in commercial pasteurised milk demonstrates that the technology, properly applied, eliminates the pathogen from the final product. The risk concentrates in raw milk, which is sometimes consumed directly or used in traditional dairy products, exposing vulnerable populations to strains that are simultaneously highly virulent and increasingly drug-resistant. The authors call for continuous monitoring of L. monocytogenes, rigorous hygiene in raw milk production, judicious antibiotic use in veterinary medicine, and routine molecular surveillance to prevent the circulation of pathogenic and resistant strains. As antimicrobial resistance continues its global advance, studies like this one demonstrate that the front line of food safety runs directly through the dairy farm, and that the tools to track the threat, from PCR screens for virulence genes to resistance indices, are already in hand.

Subject of Research: Prevalence, virulence genes, and antimicrobial resistance of Listeria monocytogenes in the dairy supply chain

Article Title: Molecular Characterisation and Antimicrobial Resistance Patterns of Listeria monocytogenes From Dairy Sources

Article References: Heidari, M. R., Shakerian, A., Chaleshtori, R. S., & Rahimi, E. (2026). Molecular Characterisation and Antimicrobial Resistance Patterns of Listeria monocytogenes From Dairy Sources. Veterinary Medicine and Science, 12(6), Article e71265. https://doi.org/10.1002/vms3.71265

Image Credits: AI Generated

DOI: 10.1002/vms3.71265

Keywords: Listeria monocytogenes, raw milk, foodborne disease, antimicrobial resistance, virulence genes, pasteurisation, dairy safety, PCR, MAR index, listeriosis, Iran, public health