Bacteria develop resistance to antibiotics through several mechanisms: 1) Decreased drug uptake into cells and increased efflux out of cells via pumps to reduce intracellular drug concentration. This is particularly important in Gram-negative bacteria. 2) Expression of resistance genes coding for altered drug target sites that have lower binding affinity for antibiotics. For example, genes for altered penicillin-binding proteins confer methicillin resistance. 3) Covalent modification of drugs via enzymes like beta-lactamases that inactivate beta-lactam antibiotics. 4) Increased production of competitive inhibitors that block antibiotic binding sites, such as the PABA inhibitor of sulfonamides. 5) Persister cells that are metabolically inactive

1. Mechanism of Bacterial
Resistance
By:- Khushboo Thakur

2. What is Bacterial Resistance ?
•Bacterial resistance is the capacity of
bacteria to withstand the effects of
antibiotics or biocides that are
intended to kill or control them.

3. Definitions
1. Antimicrobial Resistance (AMR):
• It is define as resistance of a microorganism to an antimicrobial
medicine to which it was originally sensitive.
• Resistance organisms (they include bacteria, viruses, fungi and
some parasites) are able to withstand attack by antimicrobial
medicines, such as antibiotics, antivirals, antifungals and
antimalarial.

4. (Cont.)
2. Multi-drug Resistance (MDR):
• It is define as having acquired non- susceptibility to at least one agent in
three or more antimicrobial categories.
3. Extensive drug resistance (EDR):
• It is define as non- susceptibility to at least one agent in all but two or
fewer antimicrobial categories (i.e., bacterial isolates remain susceptible to
only one or two categories).
4. Pandrug Resistance (PDR):
• it is define as non-susceptibility to all the agents in all antimicrobial
categories.

5. Mechanism of antimicrobial resistance
• Whichever way a gene is transferred to a bacterium, the
development of antibiotic resistance occurs when the
gene is able to express itself and produce a tangible
biological effect resulting the loss of activity of the
antibiotic.

6. Microbes utilize numerous mechanisms of resistance to antimicrobial
drugs the can be summarized as following:-
i. Decreased uptake and increased efflux of drugs from the
microbial cell.
ii. Expression of resistance genes that code for an altered version
of the substrate to which the antimicrobial agent binds.
iii. Covalent modification of the antimicrobial drug molecule which
inactivates its antimicrobial activity.
iv. Increased production of a competitive inhibitor of antibiotic.
v. Drug tolerance of metabolically inactive persisters.

7. 1. Decreased uptake and increased efflux of
drugs from the microbial cell.
• Decreased uptake of antimicrobial drugs and/or use of transmembrane efflux
pumps prevents the concentration of antimicrobial agent from increasing to
toxic levels within the microbial cell. (↓uptake, ↓conc, ↓effect)
• Gram –ve bacteria have an outer membrane surrounding a periplasmic space
(which contains a peptidoglycan cell wall), which surrounds an inner
membrane, whereas gram +ve bacteria have a peptidoglycan cell wall
surrounding only a single plasma membrane.
• This outer membrane may provide an extra barrier against drug uptake
(especially hydrophobic drugs) in gram –ve bacteria, which is not present in
gram +ve bacteria.
[this is the one reason why gram –ve bacteria are less susceptible than gram +ve bacteria to many
antibiotics, including beta-lactams and macrolides]

8. (Cont.)
• E.g., P. aeruginosa and E. coli are containing proton-dependant
efflux pump which expel the drug outside the cell.
• Examples:
o Fluroquinolon resistance by decrease uptake.
o Vancomycin resistance by increase thickness of bacterial cell wall, so ↓se
uptake.

9. 2. Expression of resistance genes that code for an
altered version of the substrate to which the
antimicrobial agent binds.
• Gene mutation → altered protein (substrate) → Low binding affinity → reduce
antimicrobial activity → resistance developed.
• Examples:
o MacA resistance gene coding for PBP2A (altered form than wild-type), represent
resistance of MRSA against B- lactams.
o VanA resistance gene coding for altered binding substrate (D-alanine-D- lactate ligase),
Vancomycin has 1000 times lower affinity for D-alanine-D- lactate than D-alanine-D-
alanine, so the VanA confers resistance to Vancomycin. Both Vancomycin resistant
Enterococcus (VRE)and Vancomycin resistance S. aureus (VRSA)express VanA.

10. 3. Covalent modification of the antimicrobial drug
molecule which inactivates its antimicrobial activity.
• Microbes can also express drug resistance gene that code for enzymes that
covalently modify the antimicrobial drug, thereby reducing its antimicrobial
activity.
• Examples:
o Beta-lactamase hydrolyse the beta-ring of beta-lactams, thereby inactivating the antibiotic
activity of beta-lactam molecule and conferring beta-lactam resistance.
o ACT N-acetyltransferase , which acetylates NH2 group of the aminoglycoside molecule.
Penicilliln

11. 4. Increased production of a competitive
inhibitor of antibiotic.
• Bacteria can also achieve antibiotic resistance by synthesizing a molecule
that is a competitive inhibitor of the antibiotic (Enzyme substrate).
• Example:
o Mechanism of sulphonamide resistance is increased synthesis by bacteria of Para-
aminobenzioc acid (PABA), which competes with the sulphonamides drug for the
binding site of bacterial dihydropteroate synthetase.
o This mechanism of sulphonamide resistance is used by S.aureus and N.megnintidis.

12. 5. Drug tolerance of metabolically inactive
persisters.
• The presence of metabolically inactive persisters at the site of
infection in close to actively bacterial population, results in
antibacterial tolerance.
• Recurrence of infection after treatment is usually occur.
• This mechanism occur due to expression of gene called Toxin-
Antitoxin, which cause their metabolic activity to slow or stop.
After the host exposed to antibacterial agent, the actively metabolic
bacterial of population eradicated.
The persisters are turn to metabolically active and cause recurrence of
infection.