Bacteria Infection: Definition, Types and Therapy

Bacterial infections have always been a challenge in clinical practice. From simple skin infections to severe sepsis, various types of bacterial infections can pose a serious threat to human health.

What are bacteria?

Bacteria are unicellular organisms, and their basic structure includes cell wall, cell membrane, cytoplasm and nucleosomes. More complex bacterial characteristic structures also include special parts such as capsule, flagella, pili, and cilia. The average diameter of such microorganisms varies roughly between 0.5 and 5 μm, making individual bacteria almost invisible to the naked eye.

Bacteria come in a wide variety of forms, the three most common being spherical, rod-shaped, and spiral-shaped. According to the way they ingest energy sources, they can be divided into two types: autotrophic and heterotrophic. Autotrophic bacteria have the ability to supply inorganic carbohydrates such as carbon dioxide and carbonate as the main carbon source, while relying on inorganic nitrogen, ammonia or nitrate ammonia sources, and then independently generate organic substances to meet their own physiological needs. This type of bacteria has distinct specialized properties, such as nitrobacter can only perform nitrite oxidation.

Heterotrophic bacteria, on the other hand, rely on existing organic compounds for essential nutrients. This type of bacteria can be subdivided into two categories: saprophytic bacteria and parasitic bacteria. Saprophytic bacteria feed on animal and plant remains and rotten food. Parasitic bacteria depend on a living host for essential nutrients.

Among all the bacteria that cause infectious diseases, most of them come from heterotrophic bacteria, especially parasitic bacteria. Those that have not been properly treated urban wastewater, livestock farms and slaughterhouse wastes are rich in a lot of organic matter, which is extremely suitable for the growth and reproduction of heterotrophic bacteria. In such places, the variety and number of bacteria tend to be higher than in other places, so the risk of infection naturally increases.

How to cause bacterial infection?

Although bacteria are common in our daily environment and on the surface of our skin, they usually do not cause persistent infections. For bacteria to cause an infection, they must first break through the skin or mucosal barrier and enter the bloodstream. Healthy skin and mucous membranes form an effective defense against the invasion of most bacteria and prevent the spread of infection. Once these physical barriers are compromised, the risk of bacterial infection increases significantly.

Bacteria that successfully penetrate the barrier must then escape attack by white blood cells. Antibodies in the blood recognize and mark invaders, activate an immune response, and direct white blood cells to respond quickly and engulf pathogens, thereby inhibiting bacterial proliferation and their potential spread of disease. However, the likelihood of infection increases in individuals with immature, compromised immune systems, or in the presence of new, antigenically different bacteria. In such cases, the bacteria are more likely to bypass the host's immune surveillance mechanisms, leading to infection.

Types of bacterial infections

Respiratory tract infection is a kind of common bacterial infection, among which Streptococcus is an important pathogenic factor of pharyngitis (commonly known as "strep throat"). Streptococcus pneumoniae is one of the main causes of pneumonia, which is characterized by lung infection, accompanied by cough, fever and difficulty breathing. In addition, mycoplasma can also cause atypical pneumonia, although the symptoms of this type of pneumonia are usually mild, but under certain conditions can become severe.

The digestive system is also susceptible to bacterial infections, and Salmonella and Shigella are two common bacteria that cause food poisoning. These bacteria usually enter the body with contaminated food or water, causing stomach pain, diarrhea, vomiting and fever.

Skin infections are also common bacterial infections. Staphylococcus aureus is one of the common causes of skin and soft tissue infections, including boils, abscesses and cellulitis. It is worth noting that some of the more resistant staphylococcus aureus strains, such as methicillin-resistant Staphylococcus aureus (MRSA), complicate clinical management due to their high resistance to antibiotics and often require special treatment regimens.

Urinary tract infections (UTI) are mainly caused by bacteria (such as Escherichia coli) invading the urinary system. Women have a higher incidence of UTI than men, and typical symptoms include frequent urination, urgency, painful urination, and abnormal urine color. If timely and effective intervention is not obtained, the infection may spread upward to the kidney area, and then pose a greater threat to the patient's health.

Bacterial meningitis is a rapidly progressing and potentially fatal disease. Common causes include Neisseria meningitidis and Streptococcus pneumoniae.

Antibiotics in bacterial infection therapy

Antibiotics are the primary treatment for bacterial infections, which are reagents that kill or inhibit the growth of bacteria. Here are some common antibiotics and their mechanisms of action:

Penicillin: Penicillin kills bacteria by disrupting the formation of cell walls. It can be prescribed for pharyngitis, pneumonia and skin infections.

Cephalosporins: Cephalosporins are broad-spectrum antibiotics that are used to fight various bacteria. It is widely applied to respiratory tract, urological and skin infections.

Erythromycin: Erythromycin inhibits bacterial protein production, and is commonly used for respiratory and skin infections.

Tetracycline: Tetracycline inhibits bacterial protein synthesis and is often used to treat respiratory infections, urinary tract infections, and acne.

Fluoroquinolones: Such as ciprofloxacin, which works by interfering with bacterial DNA replication and repair, are often used to treat complex urinary tract and gastrointestinal infections.

Antibiotics at BOC Sciences

Antibiotic production services at BOC Sciences

Progress in bacterial infection therapy

Due to the overuse of antibiotics, almost all antibiotics in clinical use have developed resistance in different strains, and it is becoming increasingly difficult to treat such bacterial infections. According to the World Health Organization, antibiotic-resistant bacterial infections are directly responsible for 1.27 million deaths a year. Based on this, phage therapy, antimicrobial peptides, probiotics and microbiome therapies, vaccination and other methods are used to treat and prevent bacterial infections. And new treatment strategies for bacterial infections are also being researched.

Harnessing the natural physiological function of cell membranes has become a promising approach to treating diseases. In particular, membranes from red blood cells and macrophages are being explored for biodetoxification applications. By fusing a natural macrophage cell membrane with an artificial lipid membrane, Zhao et al. developed a hybrid bionic nanomicrocapsule system containing a broad-spectrum antibiotic ciprofloxacin (CIP) capsule for the treatment of infected tissues. In this system, the macrophage cell membrane can insulate endotoxins and inflammatory cytokines, while the artificial lipid membrane is used to improve membrane properties, such as fluidity or insertion of bacterial anchor molecules. Based on these advantages, it was demonstrated in a mouse model of S.aureus focal infection that a single injection of biomimetic nanocapsules could effectively anchor and eliminate S.aureus in infected tissues and reduce inflammatory cytokine levels. These results indicate that natural membrane materials and artificial membrane materials have potential application value as multi-functional bionic drug delivery systems in the treatment of bacterial infections.

Pseudomonas aeruginosa is a kind of increasingly serious antibiotic-resistant bacteria. Inspired by the radical treatment of helicobacter pylori infection with bismuth subsalicylate combined with different classes of antibiotics (tetracycline, macrolides, quinolones, rifamycin, etc.), Sun et al. evaluated the antibacterial effect of the combination of bismuth subsalicylate against multi-resistant bacteria. It was found that bismuth salicylate could be combined with various antibiotics to eliminate Pseudomonas aeruginosa. The findings suggest that a synergistic combination of drugs offers a powerful alternative to address the rapid emergence and spread of antibiotic-resistant strains of bacteria.

The combination of bismuth and antibiotics showed a strong synergistic effect on Pseudomonas aeruginosa. (Xia, Y. S., 2024)

References

1. Che, J., et al. Artificial lipids and macrophage membranes coassembled biomimetic nanovesicles for antibacterial treatment. Small. 2022, 18(26): 2201280.
2. Xia, Y. S., et al. Bismuth-based drugs sensitize Pseudomonas aeruginosa to multiple antibiotics by disrupting iron homeostasis. Nature Microbiology. 2024: 1-14.