What is resistance?
Bacterial isolates have been categorized as being
susceptible or resistant to antibiotics ever since they became
available. Where the
concepts of the minimum inhibitory concentrations (MIC) and minimum
bactericidal concentrations of an antibiotic are described.
Unfortunately, making an accurate judgement about microbial
susceptibility or resistance is somewhat less straightforward than this
traditional working definition, since there is usually no simple
relationship between the MIC (or minimum bactericidal concentrations)
of an antibiotic and clinical response. Therapeutic success depends not
only on the concentration of the antibiotic achieved at the site of
infection (i.e. its pharmacokinetic behaviour) and its activity against
the infecting organisms encountered there (i.e. its pharmacodynamic
behaviour), but also on the contribution that the host's own defences
are able to make towards clearance of the offending microbes.
The decision as to whether a given bacterial isolate
should be labelled susceptible or resistant depends ultimately on the
likelihood that an infection with that organism can be expected to
respond to treatment with a given drug, but microbiologists and
clinicians have become accustomed to the idea that an organism is
‘resistant’ when it is inhibited in vitro by an antibiotic
concentration that is greater than that achievable in vivo. Importantly
the concentration of antibiotic that is achievable will vary according
to the site of infection, dosage, and route of administration. For
example, some antibiotics, such as trimethoprim, are excreted primarily
via the kidneys and therefore achieve, in the context of urinary tract
infections, advantageously high concentrations in urine. Furthermore,
the intrinsic activity of an antibiotic against some bacteria (e.g.
staphylococci) may be greater than for others (e.g. Escherichia coli)
because of the effect of cell envelope structure on achievable
intracellular antibiotic concentrations. These issues mean that several
different thresholds (breakpoint concentrations) are often used to
define susceptibility to an antibiotic. For example, an E. coli strain for which the MIC of ampicillin is 32 mg/l might be classed as susceptible if isolated from a urinary infection, while the same bacterium causing a
bloodstream infection would be classified as ampicillin resistant.
These differences in definition of susceptibility relate to the
variations in achievable concentrations at the site of infection: while
an ampicillin concentration of 32 mg/l can reliably be achieved in
urine, this is not the case in blood.
Intrinsic resistance
If whole bacterial species are considered, rather than
individual isolates, it is apparent immediately that they are not all
intrinsically susceptible to all antibiotics (See Table Below);
for example, a coliform infection would not be treated with
erythromycin, or a streptococcal infection with an aminoglycoside,
since the organisms are intrinsically resistant to these antibiotics.
Similarly, Pseudomonas aeruginosa and Mycobacterium tuberculosis
are intrinsically resistant to most of the agents used to treat more
tractable infections. Such intrinsically resistant organisms are
sometimes termed non-susceptible, with the term resistant reserved for
variants of normally susceptible species that acquire mechanism(s) of
resistance.
A microbe will be intrinsically resistant to an
antibiotic if it either does not possess a target for the drug's
action, or it is impermeable to the drug. Thus, bacteria are
intrinsically resistant to polyene antibiotics, such as amphotericin B,
as sterols that are present in the fungal but not bacterial cell
membrane, are the target for these drugs. The lipopolysaccharide outer
envelope of Gram-negative bacteria is important in determining
susceptibility patterns, since many antibiotics cannot penetrate this
barrier to reach their intracellular target. Fortunately, intrinsic
resistance is therefore often predictable, and should not pose problems
provided that informed and judicious choices of antibiotics are made
for the treatment of infection. Of greater concern is the primarily
unpredictable acquisition or emergence of resistance in previously
susceptible microbes, sometimes during the course of therapy itself.
Acquired resistance
Introduction of clinically effective antibiotics has
been followed invariably by the emergence of resistant strains of
bacteria among species that would normally be considered to be
susceptible. Acquisition of resistance has seriously reduced the
therapeutic value of many important antibiotics, but is also a major
stimulus to the constant search for new and more effective
antimicrobial drugs. However, while the emergence of resistance to new
antibiotics is inevitable, the rate of development and spread of
resistance is not predictable.
The first systematic observations of acquired drug
resistance were made by Paul Ehrlich between 1902 and 1909 while using
dyes and organic arsenicals to treat mice infected experimentally with
trypanosomes. Within a very few years of the introduction of
sulphonamides and penicillin (in 1935 and 1941 respectively),
micro-organisms originally susceptible to these drugs were found to
have acquired resistance. When penicillin came into use less than 1% of
all strains of Staphylococcus aureus were
resistant to its action. By 1946, however, under the selective pressure
of this antibiotic, the proportion of penicillin-resistant strains
found in hospitals had risen to 14%. A year later, 38% were resistant,
and today, resistance is found in more than 90% of all strains of Staph. aureus. In contrast, over the same period, an equally important pathogen, Streptococcus pyogenes, has remained uniformly susceptible to penicillin, although there is no guarantee that resistance will not spread to S. pyogenes in future years.
There is no clear explanation for the marked differences
in rate or extent of acquisition of resistance between different
species. Possession of the genetic capacity for resistance does not
always explain its prevalence in a particular species. Even when
selection pressures are similar, the end result may not be the same.
Thus, although about 90% of all strains of Staph. aureus are now resistant to penicillin, the same has not happened to ampicillin resistance in E. coli under similar selection pressure. At present, apart from localized outbreaks involving epidemic strains, about 40-50% of E. coli
strains are resistant to ampicillin, and this level has remained more
or less steady for a number of years. However, since an increasing
incidence of resistance is at least partly a consequence of selective
pressure, it is not surprising that the withdrawal of an antibiotic
from clinical use may often result in a slow reduction in the number of
resistant strains encountered in a particular environment. For example,
fluoroquinolone resistant strains of P. aeruginosa
that emerged in some hospitals as ciprofloxacin or levofloxacin were
used more frequently were replaced by more susceptible strains
following restriction of removal of these drugs. Conversely,
sulphonamide resistant E. coli strains
that became commonplace when the sulphonamide-containing combination
drug co-trimoxazole was widely used are still prevalent. This is
probably because the selection pressure still exists for other
antibiotics, such as ampicillin, and the genes coding for sulphonamide
and ampicillin resistance are often closely linked on plasmids; hence,
use of one antibiotic can select or maintain resistance to another. The introduction of new antibiotics has also resulted in
changes to the predominant spectrum of organisms responsible for
infections. In the 1960s semi-synthetic ‘β-lactamase stable’
penicillins and cephalosporins were introduced
which, temporarily, solved the problem of staphylococcal infections.
Unfortunately, Gram-negative bacteria then became the major pathogens found in hospitals and rapidly acquired resistance to multiple antibiotics in the succeeding years. In the 1970s the pendulum swung the other way with the first outbreaks of hospital infection with multiresistant staphylococci that were resistant to nearly all antistaphylococcal agents. Outbreaks of infection caused by such organisms have occurred subsequently all over the world. There are now signs that Gram-negative bacteria are once again assuming greater importance, particularly in hospitals. Resistance to newer cephalosporins—mediated by extended-spectrum β-lactamases—and fluoroquinolones in E. coli and other enterobacteria is increasing, rendering these commonly used antibiotics less effective. Multiresistant Gram-negative bacteria (such as Acinetobacter species) have emerged that are resistant to most and, occasionally, all approved antibiotics.
Types of acquired resistance
Two main types of acquired resistance may be encountered
in bacterial species that would normally be considered susceptible to a
particular antibacterial agent.
Mutational resistance
In any large population of bacterial cells a very few individual cells may spontaneously become resistant.
Such resistant cells have no particular survival advantage in the
absence of antibiotic, but after the introduction of antibiotic
treatment susceptible bacterial cells will be killed, so that the
(initially) very few resistant cells can proliferate until they
eventually form a wholly resistant population. Many antimicrobial
agents select for this type of acquired resistance in many different
bacterial species, both in vitro and in vivo. The problem has been
recognized as being of particular importance in the long-term treatment
of tuberculosis with antituberculosis drugs.
Transmissible resistance
A more spectacular type of acquired resistance occurs
when genes conferring antibiotic resistance transfer from a resistant
bacterial cell to a sensitive one. The simultaneous transfer of
resistance to several unrelated antimicrobial agents can be
demonstrated readily, both in the laboratory and the patient.
Exponential transfer and spread of existing resistance genes through a
previously susceptible bacterial population is a much more efficient
mechanism of acquiring resistance than the development of resistance by mutation of individual susceptible cells.
Here it is sufficient to stress that however resistance appears in a
hitherto susceptible bacterial cell or population, resistance will only
become widespread under the selective pressures produced by the
presence of appropriate antibiotics. Also, the development of resistant
cells does not have to happen often or on a large scale. A single
mutation or transfer event can, if the appropriate selective pressures
are operating, lead to the replacement of a susceptible population by a
resistant one. Without selective pressure, antibiotic resistance may be
a handicap rather than an asset to a bacterium.
Cross-resistance and multiple resistance
These terms are often confused. Cross-resistance
involves resistance to a number of different members of a group of
(usually) chemically related agents that are affected alike by the same
resistance mechanism. For example, there is almost complete
cross-resistance between the different tetracyclines, because
tetracycline resistance results largely from an efflux mechanism that
affects all members of the group. The situation is more complex among
other antibiotic families. Thus, resistance to aminoglycosides may be
mediated by any one of a number of different drug-inactivating enzymes with different
substrate specificities, and the range of aminoglycosides to which the
organism is resistant will depend on which enzyme it produces.
Cross-resistance can also be observed occasionally between unrelated
antibiotics. For example, a change in the outer membrane structure of
Gram-negative bacilli may concomitantly deny access of unrelated
compounds to their target sites. In contrast, multiple drug (multidrug) resistance
involves a bacterium becoming resistant to several unrelated
antibiotics by different resistance mechanisms. For example, if a
staphylococcus is resistant to penicillin, gentamicin, and
tetracycline, the resistances must have originated independently, since
the strain destroys the penicillin with a β-lactamase, inactivates
gentamicin with an aminoglycoside-modifying enzyme, and excludes
tetracycline from the cell by an active efflux mechanism.
It is, however, not always clear whether
cross-resistance or multiple resistance is being observed. Genes
conferring resistance to several unrelated agents can be transferred en
bloc from one bacterial cell to another on plasmids, thereby giving the appearance of cross-resistance.
In such cases, detailed biochemical and genetic analysis may be required to prove that the resistance mechanisms are distinct (multiple resistance), although the genes conferring resistance are linked and transferred together on one plasmid.
In such cases, detailed biochemical and genetic analysis may be required to prove that the resistance mechanisms are distinct (multiple resistance), although the genes conferring resistance are linked and transferred together on one plasmid.
The clinical problem of drug resistance
Concerns about resistance have been raised at regular
intervals since the first introduction of antimicrobial chemotherapy,
but awareness of the antibiotic resistance problem has probably never
been greater than it is today. It has been suggested that antibiotic
resistance is becoming so commonplace that there is a danger of
returning to the pre-antibiotic era. It is important not to understate
or overstate the problem; the situation is presently becoming serious,
but is not yet desperate since most infections are still treatable with
several currently available agents. This may, however, mean that the
only antibiotics that are still active are more toxic or less effective
(or both) than those to which bacteria have acquired resistance. For
example, it is generally accepted that glycopeptide antibiotics are
less effective in the treatment of Staph. aureus
infection than are antistaphylococcal penicillins (e.g.
flucloxacillin); since the latter cannot be used against
methicillin-resistant Staph. aureus
(MRSA), this may partly explain the poorer outcome that is seen in such
cases in comparison with infection caused by methicillin-susceptible
strains.
There is good evidence that if the antibiotic regimen
chosen is subsequently shown to be inactive against the pathogens
causing infection, then patient outcome is worse (See Fig. Below).
This means that clinicians are likely to opt for unnecessarily
broad-spectrum therapy particularly in critically ill patients.
Unfortunately, repeated use of such regimens against bacteria that
harbour resistance genes intensifies the selective pressure for further
resistance development, notably in hospital, where the most vulnerable
patients are managed.
In many less-developed countries of the world the
therapeutic options may be severely restricted for economic reasons.
There is no doubt that the problem of antibiotic resistance is a global
issue, and in future years there is a real possibility that physicians
will be faced increasingly with infections for which effective
treatment is not available. Some of the organisms in which resistance
is a particular problem are summarized below.
Enteric Gram-negative bacteria
The prevalence of resistance in hospital strains of
enteric Gram-negative bacteria has been rising steadily for the past 40
years, particularly in large
units. Although cephalosporins, quinolones, and aminoglycosides have
been developed to cope with the problem, resistance to these newer
compounds continues to increase in most countries. Outbreaks of
infection caused by multiresistant Klebsiella
strains and extended-spectrum β-lactamase-producing enterobacteria in
general are being reported with worrying frequency, especially in high
dependency areas of hospitals.
Widespread resistance in enteric bacteria is a
particular problem in less-developed areas of the world where heavy and
indiscriminate use of antibiotics may combine with a high prevalence of
drug-resistant bacteria in the faecal flora, poor standards of
sanitation, and a high incidence of diarrhoeal disease to encourage the
rapid emergence and spread of multiresistant strains of enteric
bacteria. Epidemics of diarrhoeal disease caused by multiresistant
strains of intestinal pathogens, including Vibrio cholerae, shigellae, salmonellae, and toxin-producing strains of E. coli, have occurred around the world.
Acinetobacter
Acinetobacter
These organisms cause hospital-acquired infections
especially in patients in intensive care units, e.g.
ventilator-associated pneumonia. Such infections are usually extremely
difficult to treat because of the multiple classes of antibiotic
resistance found in these bacteria. Very few antibiotics are now
reliably effective for treatment of acinetobacter infections. Even with
carbapenems such as imipenem, resistance has started to emerge.
Colistin, a relatively toxic old antibiotic that has largely been
abandoned for systemic administration, has been used to treat some
strains that were resistant to all other licensed antibiotics. Many
multiresistant Acinetobacter spp. strains are currently susceptible to the new antibiotic tigecycline, and this may be helpful in such infections.
Staphylococci and enterococci
MRSA is endemic in many hospitals and nursing homes. The proportion of Staph. aureus
isolates causing serious sepsis, such as bloodstream infection, that
are resistant to methicillin has reached 40-50% in the UK and some
countries in southern Europe. The prevalence is even higher in
countries in the Far East and USA. MRSA infections have often been
treated with glycopeptides, and isolates with low-level resistance to
these antibiotics can be found. Very occasional MRSA strains with
high-level resistance to glycopeptides have also been reported. Several
newer antibiotics, including linezolid, daptomycin, and tigecycline,
are active against these strains, but occasional reports of resistance
have already occurred.
Coagulase-negative staphylococci and enterococci are
often multiresistant and cause infections typically in patients with
indwelling prosthetic material, such as catheters, vascular grafts,
joints and heart valves. A combination of antibiotics may be required
to treat serious enterococcal infections, but the emergence of
high-level aminoglycoside resistance may seriously limit this option.
Enterococci carrying genes conferring high-level resistance to
glycopeptides have emerged.
Linezolid has been used successfully to treat infection caused by such strains, but resistance has some occurred in patients receiving long courses of therapy, particularly if the focus of infection has not been removed.
Linezolid has been used successfully to treat infection caused by such strains, but resistance has some occurred in patients receiving long courses of therapy, particularly if the focus of infection has not been removed.
Streptococcus pneumoniae
Another major problem concerns the emergence of resistance in Streptococcus pneumoniae,
the most common cause of community-acquired pneumonia and other
respiratory infections. This organism used to be combated easily by treatment
with penicillin and its derivatives. Unfortunately, isolates with
resistance to most antibiotics can now be found in most countries of
the world. Such infections are often treated with broad-spectrum
cephalosporins, which can attain sufficient tissue concentrations to
exceed the raised MIC for these strains. The prevalence of
macrolide-resistant pneumococci tends to correlate with how often these
antibiotics are used, especially in the community where most
respiratory tract infections are treated. Newer fluoroquinolones such
as moxifloxacin have increased activity against pneumococci. Some
resistance emergence has developed in units where these agents have
been used commonly.
Neisseria meningitidis
Decreased levels of susceptibility to penicillin have
been seen in many countries, but high-level resistance is exceptionally
rare. The emergence of resistance to penicillin in N. meningitidis
has important strategic implications because of the need for immediate
treatment of the life-threatening infections caused by these organisms.
Currently penicillin is still used empirically in some cases of
suspected meningococcal infection. The cephalosporins cefotaxime or
ceftriaxone are often favoured for the empirical treatment of
meningitis because the antibiotic concentration achieved in the
cerebrospinal fluid more reliably exceeds the MIC for the pathogen in
both meningococcal and pneumococcal infection.
Tuberculosis
Strains of M. tuberculosis
that are resistant to two or more of the first line drugs—isoniazid,
ethambutol, rifampicin, and streptomycin—are increasing common,
particularly in HIV-infected patients. The prevalence of resistant
strains varies markedly between countries: in 0-14% of new cases of
tuberculosis (median: 1.4%) and 0-54% of previously treated cases
(median: 13%) in recent World Health Organization surveys. The
resistant bacteria can be transmitted, for example in hospitals,
prisons and in the community, and represent a major public health
issue. The emergence of resistance is associated with poor compliance
with antituberculosis medication. Directly observed therapy is
increasingly advocated therefore for patients in whom compliance may be
unreliable.