Genetic characteristics and antimicrobial resistance profile of non-typeable Haemophilus influenzae from the respiratory tract in a tertiary hospital in southwest China

In the human body, the upper airways are often home to a vast community of microscopic residents. Most of these bacteria live peacefully, but some can turn dangerous when the immune system is weakened or when they move to parts of the body where they do not belong. One such bacterium is Haemophilus influenzae. For decades, the medical world focused its attention on a specific version of this germ that wears a slippery, sugary coat, known as a capsule. This coated version was responsible for severe, life-threatening infections like meningitis, and a vaccine successfully tamed it. However, once that coated version was pushed back, a different form emerged as the primary threat. This form, called non-typeable Haemophilus influenzae, lacks the protective coat. Instead, it relies on a different set of tools to stick to surfaces, hide from the immune system, and cause infections in the lungs and sinuses. Because it is so common and adaptable, it has become a leading cause of respiratory illness, particularly in older adults. The challenge for doctors is that this bacterium has learned to ignore many of the medicines designed to kill it, making infections harder to treat.

Researchers at West China Hospital in Chengdu recently set out to understand exactly how this bacterium behaves in their local region. They gathered 57 samples of the bacteria from the respiratory tracts of patients between January and April 2025. Every single one of these samples was identified as the non-coated, non-typeable variety. The patients from whom these samples were taken were mostly men, and nearly half of them were sixty years of age or older. Many of these individuals were already dealing with other health issues, such as chronic lung disease, cancer, or diabetes. The scientists did not just look at the bacteria under a microscope; they read the complete genetic instruction manual for each sample. This allowed them to see the bacteria's family history, the specific weapons it carried to fight off drugs, and the genes that helped it survive inside the human body.

The genetic analysis revealed that these bacteria were not all the same. The researchers found 41 distinct genetic families, or lineages, among the 57 samples, showing that the bacteria in this hospital are highly diverse. One family, known as ST422, was the most common, appearing in four of the samples. Despite this variety, the bacteria shared a core set of survival tools. Every single isolate carried 54 specific genes that help it manipulate the human immune system, allowing it to hide and persist. These genes act like camouflage, helping the bacteria avoid detection while they cause infection. The study also confirmed that the bacteria were well-equipped to steal iron and nutrients from their host, which is essential for their growth in the iron-poor environment of the human body.

The most critical part of the investigation was determining which medicines could still kill these bacteria. The results painted a clear picture of resistance. The bacteria were highly resistant to ampicillin, a common antibiotic, with 70.2% of the samples surviving exposure to it. Resistance was also high against a combination drug called trimethoprim-sulfamethoxazole and a version of ampicillin mixed with a helper drug called sulbactam. The bacteria achieved this resistance in two main ways. First, nearly half of the samples produced an enzyme called beta-lactamase, which acts like a pair of molecular scissors, cutting the antibiotic molecule in half before it can do any damage. All of these enzymes were of a specific type known as TEM-1. Second, and perhaps more significantly, the vast majority of the bacteria had altered the very target that the antibiotics are designed to hit. The bacteria changed the shape of a specific protein on their surface, known as penicillin-binding protein 3. This protein is essential for building the bacterial cell wall. By changing its shape, the bacteria made it impossible for the antibiotic to lock onto it, much like a key that no longer fits a changed lock.

The researchers identified 39 different patterns of these shape changes, with some specific changes appearing in more than half of the samples. When the bacteria possessed both the scissors enzyme and the altered protein target, they became extremely difficult to treat. However, the study also found a silver lining. Despite the high resistance to older drugs, every single one of the 57 bacteria remained vulnerable to a newer generation of antibiotics, including certain types of cephalosporins and a class of drugs called carbapenems. This means that while the bacteria have evolved to defeat many common treatments, there are still effective weapons available for doctors to use. The study also found that the bacteria had developed resistance to other types of drugs, such as those used for ear infections or stomach issues, but these were less common.

The genetic map of these bacteria showed two main groups. One group was closely related to each other and shared a specific pattern of protein changes that made them resistant to ampicillin. The other group was more varied, containing bacteria that lacked the scissors enzyme but still had altered protein targets. This diversity suggests that the bacteria are constantly evolving and adapting to the pressure of antibiotic use. The researchers noted that the specific patterns of resistance seen in this hospital differed from those found in other parts of the world, highlighting that local conditions and antibiotic use create unique evolutionary paths for these germs.

This work underscores the importance of keeping a close watch on how bacteria change over time. The high rate of resistance to ampicillin is driven by a combination of enzyme production and structural changes in the bacteria's cell wall. While the bacteria have become formidable against older medicines, they have not yet developed the ability to resist the newer, stronger antibiotics tested in this study. The findings suggest that doctors in this region should be cautious when prescribing older drugs but can rely on the newer generations of antibiotics for effective treatment. Continuous monitoring is essential, as the bacteria may eventually develop resistance to these newer drugs as well. By understanding the specific genetic makeup and resistance patterns of these bacteria, medical professionals can make better decisions to protect patients from infection and ensure that the right medicines are used when they are needed most.