Antibiotic resistance in Bacteria

MBBS/BDS 1st year27.10.2010

Antibiotic resistance in bacteria Emergence of antibiotic resistance is a major factor

limiting long term successful use of an antimicrobial agent.

Antibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic.

Resistant organism: One that will not be inhibited or killed by an antibacterial agent at concentrations of the drug achievable in the body after normal dosage.

If a bacterium carries several resistance genes, it is called multiresistant or, informally, a superbug or super bacteria.

Factors contributing for resistance Misuse of antibiotics<Use of antibiotics with no clinical indication (e.g, for viral

infections)

<Use of broad spectrum antibiotics when not indicated<Inappropriate choice of empiric antibiotics

Overuse of antibiotics

Addition of antibiotic to the feed of livestock

Failure to follow infection control practices

http://en.wikipedia.org/wiki/File:Antibiotic_resistance.svg

Settings that Foster Drug Resistance

< Day-care centers

< Long term care facilities

< Homeless shelters

< Jails

Community

Hospital < Intensive care units

< Oncology units

< Dialysis units

< Rehab units

< Transplant units

< Burn units

Settings that Foster Drug Resistance

Antibiotic resistance in bacteria Two types:

Intrinsic: Naturally occuring trait Species or genus specific

Acquired: Acquired resistance implies that a susceptible organism has

developed resistance to an agent to which it was previously susceptible, and can occur in two general ways: by mutation (s) in the existing DNA of the organims or by acquisition of new DNA.

Present in only certain strains of a species or of a genus

Genetics of ResistanceMutational resistance:

A single chromosomal mutation may result in the synthesis of an altered protein: for example, streptomycin resistance via alteration in a ribosomal protein, or the single aminoacid change in the enzyme dihydtropteroate synthetase resulting in a lowered affinity for sulfonamides

A series of mutations, for example, changes in penicillin binding proteins (PBPs) in penicillin resistant pneumococci

Genetics of Resistance Resistance by acquisition of new DNA– By Transformation– Conjugation– Transduction

Nature of elements involved in transferring DNA: Plasmids: plasmid mediated resistance much more efficient

than the resistance ass. with chromosomal mutation

Transposons

Mechanism of action of antibiotics

Folic acid synthesis

ß-lactams & Glycopeptides (Vancomycin)

50 50 5030 30 30

DNA

mRNA

Ribosomes

PABA

DHFA

THFA

Cell wall synthesis

DNA gyrase

Quinolones

Protein synthesis inhibition

Protein synthesis inhibitionTetracyclines

Protein synthesis mistranslation

Macrolides & Lincomycins

Cohen. Science 1992; 257:1064

DNA-directed RNA polymerase

Rifampin

Aminoglycosides

Sulfonamides

Trimethoprim

Mechanisms of antibiotic resistance : how DO the bacteria do it ??

Mechanisms of resistance (Contd.)2. Alteration of Access to the target site (altered uptake or increased exit)

Involves decreasing the amt of drug that reaches the target by either: Altering entry, for example, by decreasing the permeability of the cell wall, Pumping the drug out of the cell (known as efflux mechanisms)

3. Enzymatic inactivation:

Enzymes that modify or destroy the antibacterial agent may be produced (drug inactivation)

e.g., Beta lactamasesAminoglycoside modifying enzymesChloramphenicol acetyl transferase

4. Bypass of an antibiotic sensitive steps

Mechanisms of resistance:

Resistance mechanisms can be broadly classified into 4 types:

1. Alteration of the target site – The target site may be altered so that it has a

lowered affinity for the antibacterial (antibiotic), but still functions adequately for nomal metabolism to proceed. Alternatively, an additional target (e.g enzyme) may be synthesized.

Mechanism of resistance to particular antibiotics

Resistance to β -lactams:

Resistance due to β -lactamases: most prevalent

Alteration in the pre-existing penicillin binding proteins (PBPs)

Acquisition of a novel PBP insensitive to beta β –lactams: e.g, methicillin resistance in Staphylococcus aureus (MRSA)

Changes in the outer membrane proteins of Gram negative organisms that prevent these compounds from reaching their targets

Aminoglycoside Resistance:

Intrinsic and acquired resistance due to decreased uptake

Acquired resistance is frequently due to plasmid encoded modifying enzymes:

Three classes of aminoglycoside modifying enzymes: Acetyltransferases, Adenyltransferases and Phosphotransferases

Ribosomal target modification

Tetracycline resistance

Most common antibiotic resistance encountered in nature

Mechanisms: Altered permeability due to chromosomal mutations Active efflux or Ribosomal protection (by

production of a protein) resulting from acquisition of exogenous DNA

Macrolide, Lincosamide and Streptogramin resistance:

Intrinsic resistance is due to low permeability of outermembrane protein

Acquired resistance occurs most often by alteration of the ribosomal target

Drug inactivation and active efflux may also occur

Chloramphenicol resistance

Enzymatic inactivation:– From acquisition of plasmids encoding

chloramphenicol acetyl transferase

Decreased permeability:

Quinolone resistance

Alteration of target i.e, DNA gyrase (by mutation in gyrA gene)

Decreased permeability

Glycopeptide resistance

Alteration of targete.g, Vancomycin resistance in Enterococci

Cotrimoxazole (Sulfonamides and trimethoprim) resistance

Intrinsic resistance: outer membrane impermeability

Acquired resistance: – Chromosomal mutations in the target enzymes

[low level resistance)– Plasmid mediated resistance: high level resistance

Resistance to antimycobacaterial agents

First line essential antituberculous agents: Rifampin, isoniazid and Pyrazinamide

First line supplemental: Ethambutol and Streptomycin

Second line: Para-aminosalicylic acid, ethionamide, cycloserine, kanamycin, amikacin, capreomycin, thiacetazone

Resistance to Rifampin:– From spontaneous point mutations that alter the beta

subunit of the RNA polymerase (rpoB) gene Resistance to Isoniazid: – Mutations in the catalase peroxidase gene or inhA

gene Resistance to Pyrazinamide: – Mutations in the pncA gene, which encodes for

pyrazinamidase

Multidrug resistance/ XDR

Some resistant pathogensStaphylococcus aureus:

Penicillin resistance in 1947 Methicillin resistance in 1961: MRSA causing carious

fatal diseases Vancomycin resistance in the recent years: As VRSA

and VISA

Enterococci: Penicillin resistance seen in 1983 Vancomycin resistant Enterococcus (VRE) in 1987 Even emergence of linezolid resistance

Some resistant pathogens (contd.)Pseudomonas aeruginosa:– One of the worrisome characteristic: low antibiotic

susceptibility– Multidrug resistance common: due to mutation or

horizontal transfer of resitant genes

Acinetobacter baumanii Multidrug resistance Some isolates resistant to all drugs

Salmonella, Esherichia coli

Mycobacterium tuberculosis

Tests for detecting antibacterial resistance

Disk diffusion method

Screening method: eg, oxacillin resistance screening for Staphylococcus, Vancomycin resitance screeening for enterococci

Agar dilution method: by determining minimum inhibitory concentration

Special tests: detection of enzymes mediating resistance- colorometric nitrocefin and acidometric method for beta lactamase detection

Limitation of Drug ResistanceEmergence of drug resistance in infections may be minimized in the following ways:

By prudent use of antibiotics; by avoiding exposure of microorganisms to a particularly valuable drug by limiting its use, especially in hospitals.

By maintaining sufficiently high levels of the drug in the tissues to inhibit both the original population and first-step mutants;

By simultaneously administering two drugs that do not give cross-resistance, each of which delays the emergence of mutants resistant to the other drug (eg, rifampin and isoniazid in the treatment of tuberculosis); and

By institution of infection control practices

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