Antimicrobial Resistance and Virulence Profiles of Salmonella spp. Isolated from Fresh Vegetables in Traditional Markets in Ogun State, Nigeria

The food on our plates is a complex ecosystem, often carrying invisible passengers that can make us sick. Among the most common culprits are bacteria called Salmonella, which cause millions of infections worldwide every year. While we often think of these germs coming from meat or eggs, they can also hitch a ride on fresh vegetables. These plants can become contaminated at many points, from the water used to irrigate them in the field to the way they are handled and stored in the market. A growing worry for scientists and doctors is that these bacteria are becoming harder to kill. This happens when bacteria develop resistance to antibiotics, the medicines we use to treat infections. When a bacterium becomes resistant, the standard drugs stop working, leaving patients with fewer options and longer illnesses. Understanding how these resistant bacteria move through our food supply is essential for keeping people safe.

In a recent study focused on Ogun State in Nigeria, researchers set out to investigate this exact problem. They wanted to know if fresh vegetables sold in traditional markets were carrying Salmonella, and if so, whether those bacteria were resistant to medicine and possessed specific traits that make them dangerous. The team collected 509 samples of fresh produce, including tomatoes, carrots, and cucumbers, from traders across nine different markets. These markets represent the three main regions of the state, and the samples were gathered over several months to capture changes that might happen with the seasons. The researchers took the vegetables to the lab, where they carefully searched for the presence of Salmonella bacteria. Once they found the bacteria, they did not stop there. They tested the bacteria against a wide range of antibiotics to see which medicines could still kill them. They also looked inside the bacteria's genetic code to find specific genes that might explain why some bacteria were resistant to drugs or particularly good at causing disease.

The results showed that Salmonella was indeed present in the vegetables, confirming that these fresh foods can be a source of infection. The bacteria were found in all three regions studied, but the amount varied depending on the type of vegetable and the location. Tomatoes were the most likely to be contaminated, followed by cucumbers and then carrots. The study also found that the timing of the harvest mattered. The bacteria were more common during the dry season and less common during the early rainy season, suggesting that environmental conditions like water availability and temperature play a role in how the bacteria survive and spread. This pattern aligns with the idea that limited access to clean water during dry periods might force farmers and traders to use water sources that are more likely to be contaminated.

Perhaps the most significant part of the study was the discovery of how these bacteria behave when faced with medicine. The researchers tested the bacteria against thirteen different antibiotics. They found that the bacteria were not equally resistant to all of them. The highest level of resistance was seen against two specific drugs: azithromycin and nitrofurantoin, with more than half of the bacteria surviving exposure to these medicines. Resistance was also found against amoxicillin-clavulanate, a common combination drug. However, the bacteria were still vulnerable to several other powerful antibiotics, including gentamicin, ceftriaxone, and meropenem. This mix of resistance and sensitivity tells a complicated story. It means that while some standard treatments might fail, other options remain effective. Yet, the presence of any resistance is a warning sign, as it shows the bacteria are evolving and adapting to the drugs we rely on.

To understand why the bacteria were resistant, the scientists looked for the genetic instructions that give them this ability. They found that many of the bacteria carried genes known to protect against a class of drugs called fluoroquinolones, which are important for treating serious infections. Specifically, they found genes that help the bacteria pump drugs out of their cells or change the way the drugs work. Even though the bacteria showed only low levels of resistance to these drugs in the lab tests, the presence of these genes is concerning. It suggests that the bacteria have the potential to become fully resistant if they are exposed to the drugs again. Additionally, the researchers looked for genes that make the bacteria more dangerous to humans, known as virulence genes. They found one such gene, called stn, in a few of the bacteria. This gene helps the bacteria produce a toxin that causes diarrhea and other symptoms of illness. While they did not find other specific virulence genes, the combination of resistance genes and the toxin-producing gene in the same bacteria is a serious concern.

The study concludes that fresh vegetables sold in traditional markets in this region are a real pathway for transmitting harmful and drug-resistant bacteria to people. The findings highlight that the risk is not just about the presence of the bacteria, but also about their ability to survive treatment and cause disease. The variation in contamination levels across different markets and seasons points to specific weaknesses in how the food is handled and stored. The researchers suggest that improving hygiene, ensuring access to clean water for washing produce, and better managing how vegetables are stored could significantly reduce the risk. By combining tests that show how the bacteria react to drugs with tests that look at their genetic makeup, scientists can get a clearer picture of the threat. This approach helps identify hidden dangers that might be missed by looking at the bacteria's behavior alone. Ultimately, the work underscores the need for constant monitoring and better practices to ensure that the fresh food we eat remains safe for everyone.