If you are a microbiologist or anyone from the pharmaceutical industry, you would have most probably heard the terms bactericidal and bacteriostatic antibiotics. Both groups of drugs belong to the class of antibiotics. In other words, antibiotics are classified into bacteriostatic and bactericidal based on their mechanism of action.
How are they different?
Bacteriostatic | Bactericidal | |
Mechanism of action | They prevent the growth of bacteria i.e. keep them in their stationary phase | They kill the bacteria using specific enzymes |
Can it be Reversed? | Yes | No |
Minimal concentration | Minimal Inhibitory Concentration (MIC): the minimum drug concentration that inhibits bacterial growth | Minimal Bactericidal Concentration (MCB): the concentration of the drug required to kill 99.99% of the bacteria |
Acts on? | DNA replication and protein synthesis | Cell wall synthesis |
Who can take it? | Patients with a strong immune system | Immuno- compromised patients |
Examples | Beta-lactam antibiotics (penicillin derivatives (penams) ), cephalosporins (cephems), monobactams, and carbapenems) and vancomycin | Tetracyclines, sulfonamides, spectinomycin, trimethoprim, chloramphenicol, macrolides and lincosamides |
They might differ in their mechanisms. However, it is important to note that some drugs are bactericidal to a group of pathogens, while they may be bacteriostatic to another group of pathogen.
Now that you have seen how we can differentiate the two groups of antibiotics, let’s look at how we can differentiate them experimentally.
Mechanism of action of Bacteriostatic antibiotics
Let’s understand the mechanism using an example. Tetracycline is a classic bacteriostatic antibiotic. They work on the bacterial ribosome, which plays an important role in protein synthesis. By inhibiting ribosomal activity, no new proteins can be formed. This will not essentially kill the bacteria. But, they cannot replicate, as they would need new proteins to make a new bacterial cell. It hence stalls their growth.
Mechanism of action of Bactericidal antibiotics
An example of bactericidal antibiotics is Polymixin B. It acts on the plasma membrane of the bacteria. By puncturing the plasma membrane, the contents of the cell leak out, thereby killing the bacteria. It is important for bacterial cells to keep a balance of ions between the cytoplasm and the extracellular matrix. Polymixin B disrupts this balance. They leak out not just the ions and proteins but also important components of the cell like DNA and RNA.
How can we differentiate the two groups of antibiotics?
Under invitro conditions, the determination of whether an antibacterial agent is bactericidal or bacteriostatic depends on a number of factors. They include: growth conditions, bacterial density, test duration, and extent of reduction in bacterial numbers. Among a lot of methods devised for differentiating them, an interesting method demonstrates the use of spectroscopy.
We can follow the mechanisms of bactericidal and bacteriostatic drugs just by detecting Raman bands corresponding to DNA. This is done using Raman spectroscopy. How? By observing the strength of the DNA signal. How? The bacteriostatic agent shows little effect on the DNA, while the bactericidal agent shows a decrease in the strength of the signal assigned to DNA.
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Interesting, thanks for sharing!