In the clinical context an organism is said to be resistant:

If it is not killed or inhibited by drug concentrations

readily attainable in the patient; this usually means blood

and tissue concentrations.

However, an organism resistant to these may of course

be sensitive to the higher concentrations attainable in

urine or by topical application.

Even the broadest of broad-spectrum antibacterial drugs

is ineffective against some bacterial genera, against some

species of other genera, and usually against some strains

of species that are in general sensitive to it.

ResistanceResistance

Inherent( non

specific)

Inherent( non

specific)AcquiredAcquired

Certain bacteria are, and as far as we know always have

been, more or less resistant to some antibiotics.

For example, gram-negative bacteria, especially

Ps. aeruginosa, are inherently resistant to a number of

antibiotics that are very effective against gram-positive

bacteria such as penicillin G, erythromycin, lincomycin…….

Inherent )non specific( Resistance

When a bacterial population adapts to the presence of

an antibiotic, sensitive cells are gradually replaced by

resistant cells as in the presence of antibiotic.

Resistant cells continue to grow at the expense of

sensitive cells.

When a new antibiotic is introduced into clinical

practice for the treatment of infections caused by

bacteria that are not inherently resistant to the drug,

the majority of infections respond to the new drug.

Acquired Resistance

But following months or years of continuous use,

resistant strains are reported.

The degree of resistance and the speed with which it

develops varies with:

The organism & The drug.

Generally, the development of acquired bacterial

resistance is common and must be usually expected

with some exceptions.

Streptococcus pyogenes has remained sensitive to

Penicillin G after 40 year's exposure to the drug.

Staph aureus develops slow or multisteps resistance to

penicillin, chloramphenicol and tetracycline.

While Mycobacterium tuberculosis and various

organisms develops sudden or one step resistance to

Streptomycin.

Biochemical Mechanisms of

ResistanceProduction of drug-

inactivating enzymes

Change in the antibiotic target site

.Reduction in cellular permeability to the

antibiotic:

Switch to alternative metabolic pathways

unaffected by the drug:

Increased production of essential metabolite

Production of drug-inactivating enzymes

Inactivation of

Aminoglycosides

Inactivation of ß-lactams

Inactivation of Chloramphenicol

Inactivation of aminoglycosides by plasmid controlled

adenylating , phosphorylating or acetylating intracellular

enzymes of drug resistant gram negative bacteria.

Although the inactivating enzymes vary considerably in

their substrate and specificities, known modifications are

restricted to acetylation of amino groups and

adenylylation or phosphorylation of hydroxyl groups.

Inactivation of Aminoglycosides

Inactivation of β-lactams by β -lactamases ( penicillinase, cephalosporinase ) into penicilloic or cephalosporoic acid.

β -lactamases are:

Inactivation of ß-lactams

Gram-positive Gram-negative

Chromosomally Plasmid

Constitutive Inducible

Penicilloic acid

The synthesis of gram positive β-lactamase is induced by

the antibiotic themselves and is released extracellularly

and destroy antibiotic in the external environment.

Most strains of gram-negative cells, by contrast,

synthesize β-lactamases constitutively. i.e. continuously

and are not released into the external environment (cell-

bound or intracellular).

Chromosomally-mediaied β-lactamases of gram negative

hydrolyze cephalosporins more rapidly than penicillins and

are inhibited by cloxacillin but not by clavulanic acid.

However, those of Aeromonas spp. and

Klebsiella spp. are more active against

penicillins and not inhibited by cloxacillin.

1

2

R1= cl , R2 = H (Cloxacillin)

Inducible types are found in microorganisms such as

Pseudomonas spp., Proteus and other gram

negative bacteria but infrequently in E. coli in which, as

many enterobacter species, constitutive types could be

isolated.

Different types of plasmid-mediated β-lactamases were

isolated.

TEM type enzymes are present in almost all gram

negative bacteria.

These enzymes were first isolated from E. coli strains

isolated, in Athens, from a young girl called Temoniera

and was referred to as TEM enzyme.

Electrophoretically different type was then isolated

from Pseudomonas aeruginosa (TEM-2).

OXAE. coli

SHVKlebsiella spp.

ROBH. influenza

AERAeromonas spp.

LCRPs. aeruginosa

Inactivation of chloramphenicol by chloramphenicol

acetyl transferase (CAT).

Usually they are a plasmid-mediated enzymes which are

inducible type in gram-positive bacteria but constitutive

in gram-negative bacteria.

These enzymes acetylates the OH groups in the side chain

of the drug.

Inactivation of chloramphenicol

Replacement of the terminal - OH group of this side

chain, which is normally the first to be acetylated by an

inert fluorine atom, yields a chloramphenicol derivative

that is not susceptible to attack by CAT.

Change in the antibiotic target site

Chromosomal

Resistance to

Aminoglycosides

Resistance to

Erythromycin

Resistance- to some

Penicillins

Resistance to

Sulfonamides &

Trimethoprim

It is associated with the loss or alteration of a specific

protein in the 30S subunit of the bacterial ribosome that

serve as a binding site in the susceptible organisms.

Chromosomal Resistance to Aminoglycosides

Resistance to Erythromycin

It is associated with alteration, of its receptor on the

5OS subunit of the ribosome.

Resistance- to some penicillins due to loss or alteration

of Penicillin Binding Proteins (PBPs).

Resistance- to some Penicillins

Resistance to Sulfonamides & Trimethoprim

Occurs by alteration of the tetrahydropteroat

synthetase and tetrahydrofolate reductase,

respectively, that have a much higher affinity for

PABA than these drugs.

Bacterial cells altering the permeability of their cell

membrane making it difficult for antimicrobials to enter.

This type of resistance is found in bacteria resistant to:a. Polymyxins.b. Tetracyclines.c. Amikacin & some aminoglycosides.Streptococci have a natural permeability barrier to

aminoglycosides. This can be partly overcome by combination with cell wall

active drug, e.g. (penicillin).

Reduction in cellular permeability to the

antibiotic

The organism develop an altered metabolic pathway

that bypasses the reaction inhibited by the drug

e.g. some sulfonamide-resistant bacteria do not

require extra-cellular PABA but, like mammalian cells,

can utilize preformed folic acid.

Switch to alternative metabolic pathways unaffected by the

drug:

That is competitively antagonized by the drug in

sensitive cells e.g. resistance to sulfonamides may be

associated with high level of bacterial synthesis of

PABA.

Increased production of essential metabolite

:

The origin of drug resistance

Non Genetic Origin

Genetic Origin

This involves metabolically inactive cells or loss of target sites.

a.Most antimicrobial agents act effectively only on replicating cells.

• Mycobacteria survive for many years in tissue yet are restrained by the host's defenses and do not multiply.

• Such persisting organisms are resistant to treatment and cannot be eradicated by drugs.

• When they start to multiply they are fully susceptible to the drugs.

Non Genetic Origin

b. Loss of a particular target structure, often induced by

the drug, may result in antimicrobial resistance.

• Exposure of some gram-positive bacteria to penicillin

results in the formation of cell lacking cell wall

(i.e. L-forms).

• These cells then are penicillin resistant, having lost the

structural target site of the drug.

• When these organisms revert to their bacterial parent

forms resuming cell wall production, they are again

fully susceptible to penicillin.

Most drug-resistant microbes emerge as a result of genetic change and subsequent selection processes by antimicrobial drugs.

Genetic Origin

The mechanisms by which geneticchange occur are:

Chromosomal

Resistance

Extra Chromosom

al Resistance

This develops as a result of mutation in a gene locus

that controls susceptibility to a given antimicrobial drug.

The presence of the drug serves as a selecting

mechanism to suppress susceptible organisms and favor

the growth of drug resistant mutant.

Spontaneous mutation occurs at a frequency of 10 7 to

10 12.

Chromosomal Resistance

Chromosomal mutants are most commonly resistant by

virtue of a change in a structural receptor for a drug as

in bacterial resistance to erythromycin, lincomycin,

aminoglycosides and others by alteration of their target

site in susceptible cells.

Prevention of the emergence of resistant mutants is one

of the main indications for the clinical use of

combinations of drugs.

But provided that the mechanisms of action of the two

drugs are unrelated.

Therefore if both drugs are given in adequate dosage,

the risk of the emergence of a resistant strain is very

much less than if either is used alone.

Extra Chromosomal Resistance

Plasmids

Transposons

Bacteria often contain extra chromosomal DNA units

known as plasmids.

Some of which alternate between being free and being

integrated into the chromosome.

R factors are a class of plasmids that carry genes for

resistance to one and often several antimicrobial drugs

and heavy metals.

Plasmids

Plasmid genes for antimicrobial resistance often control

the formation of enzymes that inactivate the

antimicrobial drugs such as β-lactamases, CAT and

enzymes that inactivates aminoglycosides; or enzymes

that determine the active transport of tetracyclines

across the cell membrane, and for others.

The drug resistance (R) genes are often part of highly

mobile short DNA sequences known as transposons

(Transposable elements or jumping genes) that is able

to move, from one position to another, between one

plasmid and another or between a plasmid and

a portion of the bacterial chromosome within a

bacterial cell.

Transposons

Thus, transposons are able to insert themselves into

many different genomic sites with no homology with

them.

Simple transposons (IS) only carry information

concerned with the insertion function.

Simple transposons (IS) (i.e. insertion sequences) have

no known effects beyond transposition and

inactivation of the gene (or operon) into which they may

insert.

Complex or composite transposons (Tn) contain

additional genetic material unrelated to transposition,

such as drug-resistance genes.

Such as penicillin, kanamycin, streptomycin,

sulfonamides, tetracyclines, chloramphenicol ,and

trimethoprim.

Mechanisms of Transmission of Genetic Material and Plasmids

Transduction

Transformation

Conjugation

Transposition

This is the main mechanism for transmission of antibiotic

resistance between gram-positive cocci, and occurs in

other bacterial groups.

Plasmid DNA is enclosed in a bacteriophage and

transferred by the virus to another bacterium of the same

species e.g. the plasmid carrying the gene for β-lactamase

production can be transferred from a penicillin-resistant

to a susceptible staphylococcus if carried by a suitable

bacteriophage.

Transduction

Naked DNA passes from one cell of a species to another

cell, thus altering its genotype.

This can occur through laboratory manipulation and

perhaps spontaneously.

Transformation

Conjugation

This is the commonest method by which, multi-drug

resistance spreads among different genera of

gram-negative bacteria.

But also occurs among some gram-positive cocci.

The usual plasmid found in resistant gram-negative

bacteria consists of two distinct but frequently linked

elements:

a. One or more linked genes each conferring resistance to

a specific antibacterial drug

(resistance determinants).

b. A resistance transfer factor (RTF) that enable the cell to

conjugate with a sensitive bacterium and to transfer to

It a copy of the entire plasmid.

R-factor

The entire linked complex of RTF and resistance

determinants is known as R-factor and takes the form of

a double stranded circular molecule of DNA.

A proportion of R-factor-bearing cells (R* cells) possess

hair-like structures that extend out from the bacterial

surface known as pili.

The pili, whose synthesis is under the control of the

RTF component of the R-factor are essential to the

conjugation phenomenon with R' bacteria and the

transfer of an R- factor.

A transfer of short DNA sequences (transposon) occur

between a plasmid and another or between a plasmid

and a portion of the bacterial chromosome within

a bacterial cell.

Transposition

Transferable or infective drug resistance is important for the following reasons

i. The transferable plasmids commonly determine resistance to several unrelated drugs.

ii. Such plasmids are transferable not merely to related strains of the same species but to strains of other species and genera; for example, antibiotic-resistant but harmless organism in human or animal intestine (E. coli) can confer antibiotic resistance, by plasmid transfer, on pathogenic but previously antibiotic-sensitive bacteria of other genera which the host happens to ingest (such as typhoid or dysentery bacilli).

iii. It is possible for multiple-resistant enterobacteria to

develop in farm animals and be transmitted to man.

Development of this resistance is due to the

widespread use of antibiotics especially cheap

types, as food supplements for young animal to

accelerate their growth by partial suppression of

their intestinal flora.

a.Specific resistance: When the organism acquire

resistance to a certain drug but Its susceptibility to other

drugs is unaffected.

b. Cross resistance: Microorganisms resistant to a certain

drug may also be resistant to other drugs that share

a mechanism of action.

Specific and Cross Resistance

Such relationship exist mainly between agents that are

closely related

Polymyxin B and Polymyxin E.

Erythromycin and Oleandomycin.

Neomycin and kanamycin.

However, it may also exist between unrelated

chemicals Erythromycin-Lincomycin.

When the active nucleus of the chemicals is so similar,

extensive cross-resistance is to be expected e.g.

resistance to one of the tetracyclines imparts resistance

to the other members of the group e.g. resistance to

one sulfonamide cause resistance to hundreds of other

sulfonamides.

Abuse of antibiotics, is avoided for many reasons

including the following:

a.To prevent the emergence of antibiotic resistance.

b.To reduce the cost of antibiotic use.

c. To prevent antibiotic toxicity.

Antibiotic Policies

Unless there is a valid reason for giving an antibiotic, the patient would probably be better off without it.

General Principles of Optimal Antibacterial

Therapy 1

Treatment of Known or Suspected Infection

Prevention of Bacterial Infection

Peri-operative Prophylaxis

Patients at Special Risk

In cases when immediate drug treatment is necessary.2It is bad treatment to use broad-spectrum antibiotics when an infective condition can be treated with a more specific agent.

3

It is essential to use bactericidal and not bacteriostatic therapy.4

It is essential to use combination of antimicrobial drugs in certain situations.5

For treatment of superficial infections it is important to use either antiseptic or antibiotics which are rarely or never used systemically.

6Give enough, for long enough, and then stop treatment with the antibiotic.7To reduce the spread of microbial resistance, avoid the use of antibiotics as food supplement for animals or for preservation of human food stuffs, avoid liberation of antibiotic powders and solutions into the environment.

8

To reduce the emergence of antibiotic-resistant strains, an antibiotic policy has to be introduced for a hospital or area e.g., using antibiotics in rotation, keeping a particular antibiotics and permitting their use only on rare and special occasions, or insisting on combined therapy.

9

1. To provide broad coverage.

2. For initial (blind) therapy when the patient is seriously ill

and results of cultures are pending.

3. To provide synergism when organisms are not effectively

eradicated with a single agent alone e.g., in enterococcal

endocarditis both penicillin and an

aminoglycoside are given because their combined effect

is greater than the sum of their independent activities.

Drug combination

4. To prevent emergence of resistance, as in the treatment of

tuberculosis. Inappropriate use of combinations could result in: Antagonism it occurs when a bactericidal agent is used with a

bacteriostatic one as in penicillins plus tetracycline or

Chloramphenicol.

Sulphonamides do not antagonize penicillins, possibly because their

bacteriostatic action is too low. An increase in the number or severity of adverse reactions. Increased coast.