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From E. coli to Klebsiella pneumoniae: How do these Gram-negative bacteria affect our health?
Gram-negative bacteria are a group of bacteria, as opposed to gram-positive bacteria, characterized by a failure to retain crystal violet dye in Gram staining. The most striking feature of this type of bacteria is that its cell envelope consists of a thin layer of peptidoglycan cell wall sandwiched between the inner membrane (cytoplasmic membrane) and the outer membrane. This two-membrane system allows them to survive in a variety of life-supporting environments, including water, soil, and inside animals and plants.
Among Gram-negative bacteria, model organisms like Escherichia coli co-exist with various pathogenic bacteria (such as Pseudomonas aeruginosa, Chlamydia and Yersinia pestis), which pose significant challenges in the medical field.
The outer membrane of Gram-negative bacteria acts as a safety barrier and is resistant to many antibiotics (including penicillin), detergents that damage the inner membrane, and the antibacterial enzyme lysozyme produced by the animal's own immune system. This makes treating infections caused by this type of bacteria more complicated.
More importantly, the outer layer of the outer membrane contains a complex lipopolysaccharide (LPS), whose lipid A component can trigger a toxic reaction when bacteria are lysed by immune cells, which may lead to septic shock. Clinically, symptoms such as decreased blood pressure, respiratory failure, and lactic acidosis can be seen.
The challenge of antibiotics
In order to combat Gram-negative bacteria, scientists have developed many types of antibiotics, including aminopenicillins, cephalosporins, and combination drugs containing β-lactamase inhibitors (such as piperacillin-tazobactam), folic acid antagonists, and fluoroquinolones. These drugs are also commonly used to treat Gram-positive bacteria.
Antibiotics that specifically target Gram-negative bacteria include aminoglycosides, monocyclic beta-lactams (such as amoxicillin), and ciprofloxacin.
Characteristics of Gram-negative bacteria
Common Gram-negative bacteria have the following characteristics: the presence of an inner membrane, a thin peptidoglycan layer, an outer membrane containing lipopolysaccharide, and porins. These characteristics allow them to survive stably within their ecosystem.
Classification and system development
Bacterial classification based on Gram staining reaction usually divides bacteria into Gram-positive bacteria and Gram-negative bacteria. Gram-negative bacteria can be further subdivided into multiple categories, including decades of scientific research to understand the evolution and characteristics of this type of bacteria. However, this series of classification methods needs to be continuously adjusted according to the latest molecular research.
Relationship with health
Gram-negative bacteria are widely recognized as important causative agents of many diseases. Especially in medical settings, the infections they cause include pneumonia, urinary tract infections, and bacteremia, which are one of the main threats to patients' health.
Once Gram-negative bacteria enter the blood circulation system, LPS may trigger a natural immune response, ultimately leading to septic shock.
Thoughts at the end
As the problem of antibiotic resistance intensifies and the health threat of Gram-negative bacteria becomes more apparent, do we have adequate strategies to address this challenge?
