This document discusses microbial resistance to antibiotics. It notes that while some bacteria remain susceptible to antibiotics, overuse and misuse of antibiotics has led to increased resistance. Resistance can develop through natural mechanisms in bacteria or be acquired through genetic mutation or exchange between bacteria. Genetic exchange of resistance genes via plasmids and transposons allows resistance to spread between species and to multiple drugs. The document outlines various factors that influence resistance and provides recommendations to prevent further development and spread of resistance.

1. MICROBIAL RESISTANCE
 Drug resistance (It refers to unresponsiveness of a
microorganism to an AMA, and is akin to the phenomenon of
tolerance seen in higher organisms) is not a characteristic of
all bacteria and many strains responsible for common
infections have largely remained susceptible to antibiotics e.g.
pneumococci, Streptococcus pyogenes, meningococci, and
Treponema pallidum.
 The host defense, environmental factors and the properties of
the drug used influence the development of bacterial
resistance to a drug.
 Bacterial resistance is often quantitative and not qualitative.
 Thus, an antibacterial agent which is not effective in small
doses may inhibit the bacteria in vitro in large concentrations.
 This, however, may not be of much clinical significance as
such high levels can rarely be achieved in vivo owing to
possible toxicity.
 High concentrations may, however, sometimes be used for
treating resistant local infections.
 Bacterial resistance can be:
(1) Natural; or
(2) Acquired
 In organisms which are naturally resistant, (1) the drug
sensitive enzyme reactions may be absent. (2) The drug
may fail to reach the target due to permeability barrier or

2. absence of transport mechanisms or presence of efflux
pumps. (3) Some naturally resistant organisms may
elaborate a substance which destroys the antibiotic,
e.g. E. coli produce beta lactamase which destroys penicillin.
 Following the use of an antimicrobial agent which destroys
the sensitive strain, these naturally resistant variants multiply
and become dominant.
 The development of acquired microbial resistance can be
demonstrated in vitro by serially culturing the organism in
increasing concentrations of an antimicrobial drug.
 Organisms thus made resistant in vitro usually again become
susceptible to the drug following their serial subculture in the
drug free medium.
 On the other hand, the organisms which are naturally resistant
or those which develop resistance after exposure to the drug
in vivo usually retain this property.
 Microbes acquire resistance after a change in their DNA.
Such change may occur by:
Genetic mutation i.e. by alteration in the structure of their
own DNA; or more commonly
 Genetic exchange i.e. by acquisition of extra-
chromosomal DNA from other bacteria.
 In genetic mutation, the resistance is of a low level, to a single
drug, and involves nonenzymatic mechanisms such as
decreased permeability to the antibiotic e.g. methicillin
resistance of Staph. aureus. Such resistance is not transferable
to other bacteria.

3.  In some cases, the mutant bacteria even possess the capacity
to multiply in the presence of higher concentrations of the
drug concerned e.g. streptomycin-resistant mutants.
 Genetic exchange is the most important cause of serious
clinical drug resistance because it can produce epidemic
resistance to multiple drugs.
 In genetic exchange, the resistance genes are transferred from
one bacterial species to another by means of discrete,
movable, extrachromosomal DNA elements called plasmids.
 The plasmids that encode for resistance to anti-microbial
drugs are called R-plasmids or R-factors.
 A plasmid can reproduce itself and in the case of plasmid
transfer, the replicating plasmid donates one copy to the new
recipient cell while the donor plasmid retains its own copy.
 Transfer of R-plasmids between bacteria can occur by •
Conjugation, i.e., direct physical mating between
bacteria.
 Transduction, i.e., through the agency of
bacteriophages.
 Transformation where resistance genes from
chromosomes may be transferred directly through a sex
pilus without the mediation of a vector (plasmid or
phage). Examples of microbes where such transfer is
seen are streptococci, pneumococci, and some species of
hemophilus, clostridia and bacteroids.
Transposition occurs independent of R-plasmids.
Resistance determinants (transposons) have the ability to

4. ‘hop’ from one plasmid to another plasmid, or to a
chromosome, or to a bacteriophage.
 After entry into the host cell, the plasmid and the phage may
be lost, leaving the transposons permanently in the plasmids
or the chromosomes of the host cell.
 Since transposons may carry multiple resistances, the
recipient bacteria can acquire resistance to multiple drugs.
 Plasmid (R-factor) and/or transposon mediated resistance
results in:
 Decreased bacterial cell wall permeability to the antibiotic,
e.g., resistance to penicillin and chloramphenicol.
 Active extrusion of the antibiotic from the bacterial cell,
e.g., resistance to tetracycline.
 Extracellular inactivation of the antibiotic, e.g., enzymatic
degradation or modification of penicillin, cephalosporins,
aminoglycosides and chloramphenicol by resistant strains.
 Intracellular inactivation of a small amount of the
antibiotic; this inactivated part then binds to the bacterial
ribosomes and prevents them from taking up the active
drug e.g., resistance to aminoglycosides.
 Change in the bacterial ribosomes which are no longer
susceptible to the action of the antibiotic e.g., resistance to
macrolides.
 Synthesis of a drug-resistant enzyme in place of the drug-
sensitive enzyme in the biosynthetic pathway of the
bacterial cell e.g., resistance to sulfonamides and
trimethoprim.

5.  Fortunately, in the absence of further exposure to the drug
involved, the R-factor mediated resistance is often
spontaneously lost within weeks or months of its
acquisition.
 In contrast, the resistance due to genetic mutation is liable
to be permanent. R factors are found in intestinal bacteria,
especially in E. coli, Enterobacter aerogenes,
K.pneumoniae, Salmonella, Shigella, Proteus, and
Pseudomonas. Since R factors can be transferred from one
bacterial species to another, regardless of pathogenicity,
they can be transferred from resistant E. coli to Salmonella
or Shigella within the human bowel. In the case of some
bacteria the R-factor may carry up to 8-10 different R-
determinants, each responsible for resistance to a different
drug.
 Clinically, resistant Gram negative bacteria carrying R
factors present a serious problem because their resistance
to multiple drugs can spread in epidemic proportions.
 In Mexico, MDR Salmonella typhi produced the worst
epidemic of typhoid fever in modern history; and MDR
resistant S. typhi strains have been isolated from India.
Transferable drug resistance is frequent in Gram negative
bacilli causing urinary infections. The other examples of
microbes where plasmid mediated transfer is known to
occur are pneumococci, penicillin resistant H.influenzae
and N. gonorrhoeae.

6.  Organisms that become resistant to one drug may exhibit
cross resistance to other related compounds, e.g., cross
resistance is seen between kanamycin and neomycin,
between erythromycin and triacetyloleandomycin. It is
known that resistance genes and plasmids were present in
bacteria even before the advent of antibiotics.
 They would appear to be one of the mechanisms of natural
selection and survival of the bacteria. It is likely that in
their struggle for survival in nature, the bacteria (those
nonpathogenic to humans and even those which are
naturally present in animals) may develop resistance
genes. Unfortunately, these genes can then be transferred
to the human pathogens with disastrous results.
 Further, the excessive and indiscriminate use of
antimicrobials in humans in the hospitals and in the
community and in animals, has encouraged the
development of resistance and has now posed a major
problem.
 Thus, many hospital strains of staphylococci, certain
strains of E. coli and pseudomonas have become resistant
to multiple commonly used antimicrobial agents.
Cross resistance :
 Acquisition of resistance to one AMA conferring
resistance to another AMA, to which the organism has not
been exposed, is called cross resistance.

7.  This is more commonly seen between chemically or
mechanistically related drugs, e.g. resistance to one
sulfonamide means resistance to all others, and resistance
to one tetracycline means insensitivity to all others.
 Such cross resistance is often complete, however,
resistance to one aminoglycoside may not extend to
another, e.g. gentamicin-resistant strains may respond to
amikacin. Sometimes unrelated drugs show partial cross
resistance, e.g. between tetracyclines and chloramphenicol,
between erythromycin and lincomycin.
Prevention of drug resistance :
 It is of utmost clinical importance to curb development of
drug resistance.
 Measures are: (a) No indiscriminate and inadequate or
unduly prolonged use of AMAs should be made.
 This would minimize the selection pressure and resistant
strains will get less chance to preferentially propagate. For
acute localized infections in otherwise healthy patients,
symptom-determined shorter courses of AMAs are
advocated.
 (b) Prefer rapidly acting and selective (narrow spectrum)
AMAs whenever possible; broad-spectrum drugs should
be used only when a specific one cannot be dete1111ined
or is not suitable.

8.  (c) Use combination of AMAs whenever prolonged
therapy is undertaken, e.g. tuberculosis, SABE, HIV-
AIDS.
 (d) Infection by organisms notorious for developing
resistance, e.g. Staph. aureus, E. coli, M. tuberculosis,
Proteus, etc. must be treated intensively.
PROPHYLACTIC USE OF ANTIMICROBIALS
 This refers to the use of AMAs for preventing the setting
in of an infection or suppressing contacted infection
before it becomes clinically manifest.
 The latter is also called 'preemptive therapy ', which
capitalizes on the small population of pathogen in the
body before the disease is manifest.
 AMAs are frequently given prophylactically, but in a
number of circumstances this is at best wasteful if not
harmful.
 The difference between treating an infection and
preventing it is that treatment is directed against a
specific organism infecting an individual patient (targeted
therapy), while prophylaxis is often against all organism
that may cause infection.
 The valid as well as improper prophylactic uses may be
categorized as:

9. 1. Prophylaxis against specific organisms: This is
generally highly satisfactory and the choice of drug is
clearcut, because it is targeted.
(a) Rheumatic fever: Benzathine penicillin is the drug
of choice for preventing infection by group A
streptococci which cause recurrences.
(b) Tuberculosis: Children. HIV positive and other
susceptible contacts of open cases need to be protected.
lsoniazid alone for 6 months is recommended.
(c) Mycobacterium avium complex (MAC):
HIV/AIDS patients with low CD4 count may be
protected against MAC infection by
azithromycin/clarithromycin.
(d) HIV infection: Health care workers exposed to blood
by need le stick injury are to be protected by tenofovir +
emtricitabine ± lopinavir or atazanavir.
The offspring of HIV positive pregnant woman can be
protected by treating the woman with tenofovir + lamivud
ine + efavirenz. After delivery the neonate should be
given syrup nevirapine for 6 weeks.
(e) Meningococcal meningitis: during an epidemic,
especia lly in contacts; rifampin/ciproAoxacin/
ceftriaxone may be used.
(f) Gonorrhoea/syphilis: before or immediately after
contact: ampicillin/ceftriaxone.

10. (g) Recurrent genital herpes simplex: Acyclovir
prophylaxis may be given when four or more recurrences
occur in a year.
(h) Malaria: Travellers to endemic areas with high trans
mission ra te many be covered by mefloquine or
doxycycline.
(i) Influenza A2 or H INI (swine flu): during an
epidemic, especially in contacts: oseltamivir.
(j) Cholera: tetracycline prophylaxis may be given to
close contacts of a case.
(k) Whooping cough: non-immunized child contact
during the incubation period: erythromycin or
azithromycin can abort clinical disease.
(I) Plague: Doxycycline prophylax is is recommended
for contacts during an epidemic.
(m) Pneumocystis jiroveci pneumonia: Transplant
recipients on immunosuppressants/lcukaemia or AIDS
patients may be protected by cotrimoxazole.
2. Prevention of infection in high risk situations such
use of AMAs may be valid and satisfactory in certain
situations, but is controversial in others.
(a) Dental extraction, tonsillectomy, endoscopies cause
damage to mucosa harbouring bacteria and induce
bacteremia. This is harmless in most subjects, but in those
with valvular defects, this can cause endocarditis.
Appropriate prophylaxis with amoxicillin or clindamycin

11. may be given few hours before to few hours after the
procedure.
(b) Catheterization or instrumentation of urinary
tract: prophylaxis with cotrimoxazole or norfloxacin
decreases the risk of urinary tract infection (UTI).
Patients with cardiac valvular lesions may be protected
with ampicillin, gentamicin or vancomycin during
catheterization.
(c) To prevent recurrences of UTI in patients with
abnormalities of the tract: cotrimoxazole or
nitrofurantoin may be given on a long-term basis since
the organism mostly is £. coli.
(d) Chronic obstructive lung disease, chronic
bronchitis: ampicillin/doxycycline/ciprofloxacin have
been used to prevent acute exacerbations; but are of
doubtful value.
(e) lmmunocompromized patients (receiving
corticosteroids or antineoplastic chemotherapy or
immunosuppressants after organ transplantation,
neutropenic patients): penicillin/cephalosporin ± an
aminoglycoside or fluoroquinolone are often used to
prevent respiratory tracL infections and septicaemia, but
incidence or superinfections is high.
3. Prevention of infection in general : This is highly
unsatisfactory in most cases and must be condemned.
Examples are:

12. (a)Neonates, especially after prolonged or instrumental
delivery.
(b) To prevent postpartum infections in the mother
after normal delivery.
(c) Viral upper respiratory tract infections: to prevent
secondary bacterial invasion.
(d) To prevent respiratory infections in unconscious
patients or in those on respirators.
Antimicrobial prophylaxis in these situations may be
hazardous. Infection by resistant organisms, fungal and
other superinfections can occur, because it is not possible
to prevent all infections, at all times, in all individuals.
Prophylaxis of surgical site infection: Surgical site infection
(SSJ) includes superficial incisional infections (e.g. stitch
abscess), deep incisional infection (of soft tissue) and organ/
space infection.
 The purpose of surgical prophylaxis is to reduce the
incidence of SS I with minimal alteration of normal
microbial flora of the host and minimal adverse effects.
 For grading the need and intensity of antimicrobial
prophylaxis, the operative wounds have been classified
into 4 categories with increasing risk of SSI (see box).
 Wound infection occurs due 10 microbial contamination
of the surgical site. It is important for the surgeon to see
that the wound left after surgery does not get infected. Use
of sterile instruments, cross-infection control measures

13. (antiseptic/disin fectant, etc.) and good surgical technique
to minimise tissue damage, haematoma and
devascularization are the primary, and often the only,
measures needed.
 However, extensive, prolonged and often combined use
of AMAs is made for prophylaxis of infection after
practically all surgeries.
 Such misuse is particularly rampent in developing
countries, probably because of unreliability of infection
control measures.
 The SSI is directly related to the number of bacteria
present in the surgical wound at the time of closure.
Systemic antimicrobial prophylaxis should be employed
only when there is clear risk of more than the critical
number of bacteria remaining in the wound at the time of
closure and occurrence of SSL.
 ln general, it is not required for clean surgery, except in
patient at special risk. Clean surgery in otherwise healthy
subjects is associated with very low risk of SSL Incidence
of postoperative infection is higher when surgery had
lasted 2 hours or more. Prophylaxis should be given for
surgeries in which a prosthesis is inserted into the bone or
soft tissue. Even clean surgery needs to be covered by
AMA in diabetics, corticosteroid recipients and other
immunocompromised subjects, infants, elderly,
malnourished and when there is extensive tissue
handling/use of electrocautery, etc.

14.  The selection of drug, dose, timing and duration of
prophylactic medication is crucial.
 It is important that the antibiotic is not started prematurely
and is not continued beyond the time when bacteria have
access to the surgical wound.
 Administration of the AMA has to be so timed that peak
blood levels occur when clot is forming in the surgical
wound, and it is present throughout the procedure.
 Thus, most of the oral drugs are given I hour before
incision, while i.v. administration just before/after
anaesthesia best ensures effective blood levels of the
AMA during surgery.
 Most AMA do not penetrate the clot once it is formed and
is older than 3 hours. Thus, late and prolonged presence of
the antibiotic in circulation serves no purpose, but can
foster resistant organisms.
 In case of prolonged surgery, the AMA may be repeated
i.v. during the procedure. Postoperative administration of
the AMA, especially after 4 hours of wound closure is
recommended only in case of contaminated and dirty
surgery, in which case it may be given for upto 5 days.
 To be maximally effective, a relatively high dose of the
AM A is selected which yields peak blood level several
times higher than MIC for the likely pathogens. The drug
or combination of drugs is selected based on the
knowledge of the organism most commonly causing SSI
in a given procedure.

15.  Local patterns of wound infection (e.g. prevalence of
MRSA) and sensitivities of the causative organisms
should guide the selection.
 The commonly employed AMAs for prophylaxis in case
of clean and clean-contaminated surgeries are listed in the
box. Dirty contaminated wounds (including road side
accidents): The antimicrobial regimens generally
administered for 5 days in case of contaminated dirty
wounds are:
 I. Cefazolin I g i.v. 8 hourly + vancomycin I g i.v. 12
hourly.
 2. Cefoxitin I g i.v. 6 hourly/ceftizoxime I g i.v. 12 hourly.
 3. Clindarnycin 0.6 g i.v. 8 hourly + Gentamicin 80 mg
i.v. 8 hourly.
 4. Ampicillin 2 g i.v. 6 hourly/vancomycin 1 g i.v. 12
hourly + Gentamicin 80 mg i.v. 8 hourly + Metronidazole
0.5 g i.v. 8 hourly.
 5. Amoxicillin I g + Clavulanate 0.2 g i.v. 12 hourly. All
given for 5 days.

16. REFERENCES :
 Essentials of Medical Pharmacology by K D Tripathi,
8th
edition, Page no:742-744
 Pharmacology and Pharmacotherapeutics by R S
Satoskar, Nirmala N. Rege, S D Bhandarkar , 24th
edition, Page no:1103-1105.