Hugo W.B., Russel A.D.(ed). Pharmaceutical Microbiology
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to vancomycin only. The VanA phenotype is conferred by a transposon which harbours
nine genes coding for resistance (Fig. 9.4). The transposon is transferred by conjugative
plasmids. VanA and VanH are essential for the expression or resistance, which is due to
a modification of the peptidoglycan pathway to produce precursors with reduced affinity
for glycopeptides. VanA is a ligase which catalyzes the synthesis of D-alanyl-D-lactate
depsipeptide instead of D-alanyl-D-alanine. VanH is a dehydrogenase which catalyses
the synthesis of D-lactate as the substrate for VanA. The glycopeptides have much
reduced affinity for the depsipeptide. VanR and VanS are regulatory proteins which
allow expression of the other resistance genes in the presence of glycopeptides. The
VanX and VanY enzymes are responsible for removing D-alanyl-D-alanine dipeptides
from precursors and peptidoglycan to increase levels of resistance. It should be noted
that VanX is essential for resistance. The open reading frames (ORF1 and ORF2) code
for transposition functions. The function of VanZ is unclear but is thought to have a
role in the expression of high level teicoplanin resistance.
VanB-type has been less well-characterized but essentially operates in a similar
manner to VanA. Both inducible and constitutive forms of resistance have been
described, but the reasons for susceptibility to teicoplanin are unclear.
The origins of the glycopeptide-resistance genes are unknown and share little
homology with genes found in intrinsically glycopeptide-resistant organisms.
3.3.3 Fosfomycin
Fosfomycin inhibits pyruvil transferase, which is an enzyme involved in peptidoglycan
synthesis. Two mechanisms of acquired resistance have been described for fosfomycin
(Davies 1994).
Plasmid- or transposon-mediated resistance occurs by inactivation of the antibiotic.
Fosfomycin is combined with glutathione intracellularly to produce a compound lacking
in antibacterial activity. The gene encoding the enzyme catalysing this reaction has
been designated/or-r.
A second mechanism of acquired resistance to fosfomycin involves chromosomal
mutations in sugar phosphate uptake pathways which are responsible for transporting
fosfomycin into the cell. The alterations decrease accumulation of the antibiotic to
levels below those required for inhibition.
3.4 Membrane-active antibiotics
3.4.1 Polymyxins
Polymyxins are a group of antibiotics which disrupt bacterial cell membranes. Two
mechanisms of acquired resistance to the polymyxins have been identified (Russell &
Chopra 1996).
Acquired resistance to polymyxins in E. coli occurs because of chromosomal
mutations which cause incorporation of aminoethanol and aminocarabinose in lipo-
polysaccharide (LPS) in place of phosphate groups. The altered LPS has a decreased
ionic charge which results in lowered binding of polymyxin and thus an increase in
resistance to this group of antibiotics.
Bacterial resistance to antibiotics 195
The mechanism of acquired resistance in Pseudomonas aeruginosa is different.
Chromosomal mutations result in the increase of a specific outer membrane protein
with a concomitant reduction in divalent cations. Polymyxins bind to the outer membrane
at sites normally occupied by divalent cations, and therefore it is thought that a reduction
in these sites will lead to decreased binding of the antibiotic with a consequent decreased
susceptibility of the cell.
3.5 Multidrug resistance pumps
Some of the previous sections have described the acquisition of low-level resistance to
various antibiotics by alterations in the cell membrane causing decreased uptake of the
drugs. These have normally have characterized as changes in components such as porins
which result in a decrease of penetration by antibiotics.
Acquired low-level resistance to many unrelated antibiotics by efflux has also
increased in prominence in recent years {Cohtnetal. 1993, George 1996). For example,
MAR (multiple antibiotic resistance) mutants were first described in the early 1990s in
E. coli and were resistant to low levels of chloramphenicol, tetracyclines, rifampicin,
penicillins and quinolones, due to impaired uptake of the antibiotics. Increased active
efflux of the drugs has been shown to be important in this type of resistance. These
efflux pumps are normally regulated and inducible in response to external stimuli, and
often mutations causing constitutive expression are responsible for the resistance
phenotype. A number of multidrug resistance pumps (MDRs) have been identified and
are widespread among bacteria. For example, seven distinct MDRs have been described
in E. coli alone. The most common type belongs to a group of proteins involved in
membrane translocation. This type of MDR is closely related to specific efflux proteins
such as that responsible for tetracycline resistance. The origins of MDRs are unknown
but a number of factors suggest that they may have arisen by mutations in specific drug
efflux pumps causing a loss of specificity. These factors include the similarity of some
MDRs to specific drug efflux pumps such as tetracycline, and the high incidence of
apparently independent evolution of MDRs.
3.6 Antibiotics with other resistance mechanisms
3.6.1 Bacitracin
Acquired resistance to bacitracin has been observed in laboratory strains of Staph.
aureus, but resistance has been unstable and no resistant mutants have yet been isolated
in vivo. Gram-negative bacteria are intrinsically resistant to bacitracin, which inhibits
the transfer of pentapeptide units to petidoglycan.
3.6.2 Antimycobacterial drugs
The advent of multidrug resistant strains of Mycobacterium tuberculosis (MDR-TB)
has led to increased fears of untreatable infections by serious pathogens. Rifampicin,
streptomycin and, occasionally, the quinolones are drugs used in the treatment of
mycobacterial infections and resistance to those agents is as described previously. There
196 Chapter 9
are some drugs, important in the antimicrobial treatment of tuberculosis, where resistance
has arisen in MDR-TB strains, but where the mechanisms are unclear. These drugs
include isoniazid, pyrazinamide and ethambutol (Musser 1995).
There are two mechanisms of acquired resistance to isoniazid which have been
proposed. The first suggests that mutations in the katG gene inhibit the metabolism of
isoniazid into an active form which inhibits an essential protein, InhA, in mycobacteria.
The mechanism of action of isoniazid remains theoretical and therefore the mechanism
of resistance also must remain so. Nevertheless, genetic studies with laboratory strains
would tend to support the above. Mutations in the gene inhA can also confer isoniazid
resistance (and to the related drug ethionamide), presumably by reducing affinity of
isoniazid metabolic by-products for InhA. However, clinical strains of M. tuberculosis
resistant to isoniazid but with unknown mechanisms unrelated to katG or inhA have
been isolated.
Pyrazinamide is a structural analogue of isoniazid and is converted to the active
acid derivative intracellularly by a nicotinamidase. Pyrazinamide resistance has been
linked to reduced levels of nicotinamidase but the genetic determinants of resistance
have not been fully elucidated.
Mutations resulting in ethambutol resistance can arise spontaneously. The exact
changes are unknown but may involve enzymes in carbohydrate synthesis pathways.
4 The problem of antibiotic resistance
Antibiotic resistance is becoming a cause for increasing concern and is the most common
cause of treatment failure in bacterial infectious diseases (Tenover & Hughes 1996;
Tenover & McGowan 1996). Furthermore, infections with antibiotic-resistant bacteria
inevitably lead to the use of more expensive and often more toxic drugs, increased
length of infection and subsequent hospital stay, and, of course, increased costs. It
must be pointed out that at this time, the majority of infections, and particularly those
caused by serious pathogenes, remain susceptible to standard antibiotic treatment.
However, this should not be taken as a signal to relax and ignore the increasing problems
encountered in antimicrobial chemotherapy. The advent of MRSA must serve as a
warning to be heeded. The ability of this important pathogen to spread within, and
between, healthcare institutions is unparalleled, and the consequences to patients in
terms of morbidity and mortality can be severe. The emergence of vancomycin-resistant
enterococci (VRE) has caused great concern in the medical profession. Vancomycin
(or teicoplanin) is often the only antibiotic effective against some MRSA strains.
Acquisition of vancomycin resistance by MRSA would leave, for the first time since
the introduction of antibiotics, a serious pathogen untreatable with any existing antibiotic.
Transfer of vancomycin resistance from VRE to MRSA has already been demonstrated
in the laboratory and it is possible the emergence of clinical strains of vancomycin-
resistant MRSA will be encountered in the not-too-distant future. Some VRE strains
are already resistant to all available antibiotics, but the relatively low virulence of the
organism has meant that infections have been confined to seriously ill patients requiring
lengthy hospitalization. Other multiply antibiotic-resistant bacteria also cause serious
problems in hospitals. These tend to be Gram-negative bacteria, such as E. coli or
Klebsiella spp., but the appearance of these microorganisms tends to be cyclical.
Bacterial resistance to antibiotics 197
For example, gentamicin-resistant Klebsiella spp. caused problems in numerous UK
hospitals during the 1980s, but their frequency has decreased in the 1990s, with resistance
becoming more problematic in Gram-positive bacteria such as those mentioned above.
The problem is not confined to nosocomial bacteria. Certain community-acquired
pathogens have become resistant to key antibiotics in the 1990s. MDR-TB has already
been mentioned and the prospect of a resurgence of tuberculosis, especially in a drug-
resistant form, is truly frightening. The severity of the infection is particularly acute in
immunocompromised patients, such as those with human immunodeficiency virus
(HIV). MDR-TB made headline news in the US and affluent European countries but
the scale of the problem is several orders of magnitude greater in the developing world.
The ease of long-distance travel means that these problems have to be considered
globally and not in isolation.
Penicillin resistance in Strep, pneumoniae has also emerged in the 1990s. This
microorganism is a community-acquired pathogen causing serious diseases such as
pneumonia and meningitis. Penicillin is a mainstay in the treatment of infections caused
by this bacterium and is often used empirically for urgent treatment when such cases
are suspected. The delay in effective antimicrobial chemotherapy caused by infection
with a penicillin-resistant strain can often be fatal. The resistance rate in the UK is
relatively low, at around 6%, but this represents a significant increase since the 1980s.
In some European countries, resistance rates are as much as 40%.
The ability of bacteria to disseminate and acquire antibiotic resistance genes is
obviously a major cause of the spread of antibiotic resistance, but the factors involved
in the maintenance and evolution of resistance genes must be considered. The single
most important cause is selective pressure from the continuing use of antibiotics. Overuse
and misuse undoubtedly exacerbate the problem. For example, resistance rates in the
UK, which has relatively tight restrictions on the use of antibiotics, are considerably
lower than those in countries with more lenient approaches. The preponderance of
resistant organisms in healthcare institutions must be due to an environment where
exposure to antibiotics is continuous, and therefore hospital-acquired strains would
tend to be more resistant than community-acquired strains. The use of antibiotics in
hospitals is difficult to control since a medical practitioner confronted with an infection
will obviously treat the patient with the best tools at his or her disposal. Nevertheless,
certain measures can be implemented to reduce the unnecessary or misguided use of
antibiotics. These may include local antibiotic treatment policies, consultation with
experts in antimicrobial chemotherapy such as microbiologists or infectious disease
physicians, a local formulary with antibiotics restricted to those considered appropriate
for the local situation, and effective infection control policies. Measures in the
community might include restrictions of antibiotics to prescription-only status, which
is the case in the UK, but often not enforced or regulated as tightly in other countries,
and rational antibiotic prescribing by general practitioners. It is surprising how often
antibiotics are prescribed inappropriately for viral infections such as the common cold
or influenza.
The above addresses only part of the problem. The use of antibiotics is rife in areas
such as animal husbandry, agriculture, aquaculture and even in the oil industry to prevent
spoilage by contaminating microorganisms. A particularly pertinent example is the use
of avoparcin in animal feed for many years. Avoparcin is related to the glycopeptides
vancomycin and teicoplanin used for the treatment of infections in humans. There is
mounting evidence to suggest that enterococci, which are commensals in the gut, have
acquired resistance to avoparcin, and therefore cross-resistance to vancomycin and
teicoplanin, in animals first due to constant exposure to the antibiotic. Transfer of
resistance to human strains has resulted in the emergence of VRE. It seems that the
similarities between avoparcin and the other two glycopeptides was not recognized
initially because of their application in apparently unconnected areas. The prospect of
legislating to avoid such occurrences appears daunting, but attempts must be made
because the consequences may be disastrous.
5 Conclusions and comments
Bacterial resistance to antibiotics is often achieved by the constitutive possession or
inducibility of drug-inactivating or -modifying enzymes. This problem can, at least to
some extent, be overcome by designing new drugs that:
1 are unsusceptible to this enzyme attack; or
2 will inactivate the enzyme concerned thereby protecting susceptible antibiotics that,
in the absence of the enzyme, would be highly active antibacterially.
Some degree of success has been achieved in both aspects, but the development of new
antibiotics has concentrated on modifications to existing classes of drug rather than
using completely novel compounds. The ability of bacteria to evolve mechanisms to
surmount these derivatives should surprise us no longer, as the emergence of resistance
to all known antibiotics has proved. The variety of resistance mechanisms and their
ease of transfer is likely to overwhelm current attempts at producing 'new' antibiotics
effective against resistant microorganisms. Rational design of novel antibiotics would
require the elucidation of three-dimensional structures of essential bacterial enzymes
with clues as to the important functional domains for potential targets.
Another problem concerns the lack of penetration of many drugs into Gram-negative
bacteria. On the basis of current knowledge, it would seem logical that any design of
new agents should at least consider the need for compounds that can penetrate
the outer membrane of these cells even when there is a decrease in porins. In
this context, the development of peptides with antibacterial activity is worthy of
consideration. These are transported into cells via relatively non-specific permeases.
One such example is alaphosphin which is rapidly accumulated by, and concentrated
within, bacteria, where it is converted to L- 1 -aminoethylphosphonic acid which acts as
an inhibitor of peptidoglycan synthesis. Alaphosphin belongs to a group of compounds,
the phosphonopeptides, which are peptide mimics with C-terminal residues that simulate
natural amino acids. Their mechanism of action results from transport into the bacterial
cell followed by release of the alanine mimetic. These agents were considered as
being an important concept in designing new antibacterially active compounds, but
unfortunately these findings do not appear to have been followed by the development
of any significant new drugs. There is, however, growing interest in other antibacterial
peptides, many of which occur naturally in a wide range of eukaryotic organisms, but
their full potential has yet to be established. Despite extensive research, the design of
clinically effective antimetabolites seems to have been restricted largely to viruses,
with little, if any, research into possible applications to bacteria.
Bacterial resistance to antibiotics 199
At the present time we are faced with the real and frightening threat of a post-
antibiotic era in years to come, where our existing antibiotic arsenal will become largely
ineffective against bacterial infections.
6 References
Bush K., Jacoby G.A. & Medeiros A. (1995) A functional classification scheme for /3-lactamases and
its correlation with molecular structure. Antimicrob Agents Chemother, 39, 1211-1233.
Chopra I., Hawkey P.M. & Hinton M. (1992) Tetracyclines, molecular and clinical aspects. J Antimicrob
Chemother, 29, 245-277.
Cohen S.P., Yan W. & Levy S.B. (1993) A multidrug resistance regulatory locus is widespread among
enteric bacteria. J Infect Dis, 168, 484-488.
Davies J. (1994) Inactivation of antibiotics and the dissemination of resistance genes. Science, 264,
375-382.
George A.M. (1996) Multidrug resistance in enteric and other Gram-negative bacteria. FEMS Microbiol
Lett, 139, 1-10.
Georgopapadakou N.H. (1993) Penicillin-binding proteins and bacterial resistance to /^-lactams.
Antimicrob Agents Chemother, 37, 2045-2053.
Gilbart J., Perry CR. & Slocombe B. (1993) High-level mupirocin resistance in Staphylococcus aureus:
evidence for two distinct isoleucyl-tRNA synthetases. Antimicrob Agents Chemother, 37, 32-38.
Godfrey A.J. & Bryan L.E. (1984) Intrinsic resistance and whole cell factors contributing to antibiotic
resistance. In: Antimicrobial Drug Resistance (ed. L.E. Bryan), pp. 113-145. New York: Academic
Press.
Hooper D.C. & Wolfson J.S. (eds) (1993) Quinolone Antimicrobial Agents. Washington: American
Society for Microbiology.
Huovinen P., Sundstrom L., Swedberg G. & Skold O. (1995) Trimethoprim and sulphonamide resistance.
Antimicrob Agents Chemother, 39, 279-289.
Leclerq R. & Courvalin P. (1991) Bacterial resistance to macrolide, lincosamide and streptogramin
antibiotics by target modification. Antimicrob Agents Chemother, 35, 1267-1272.
Lewis M.J. (1989) The genetics of resistance. In: Antimicrobial Chemotherapy (ed. D. Greenwood),
2nd edn, pp. 146-152. Oxford: Oxford University Press.
Musser J.M. (1995) Antimicrobial agent resistance in mycobacteria: molecular genetic insights. Clin
Microbiol Rev, 8, 496-514.
Power E.G.M. & Russell A.D. (1998) Design of antimicrobial chemotherapeutic agents. In Introduction
to Principles of Drug Design (ed. H.J. Smith), 3rd edn, London: Gordon & Breach.
Russell A.D. & Chopra I. (eds) (1996) Understanding Antibacterial Action and Resistance. London:
Ellis Horwood.
Sanders C.C. (1992) /^-lactamases of Gram-negative bacteria: new challenges for new drugs. Clin
Infect Dis, 14, 1089-1099.
Shaw K.J., Rather P.N., Hare R.S. & Miller G.H. (1993) Molecular genetics of aminoglycoside
resistance genes and familial relationships of the aminoglycoside-modifying enzymes. Microbiol
Rev, 57, 138-163.
Tenover EC. & Hughes J.M. (1996) The challenges of emerging infectious diseases. Development and
spread of multiplty resistant bacterial pathogens. J Am Med Assoc, 275, 300-304.
Tenover EC. & McGowan J.E., Jr. (1996) Reasons for the emergence of antibiotic resistance. Am J
Med Sd,
311,
9-16.
Woodford N., Johnson A.P., Morrison D. & Speller D.C. (1995) Current perspectives on glycopeptide
resistance. Clin Microbiol Rev, 8, 585-615.
200 Chapter 9
Chemical disinfectants, antiseptics and
preservatives
Introduction
Disinfectants, antiseptics and preservatives are chemicals which have the ability to
destroy or inhibit the growth of microorganisms and which are used for this purpose.
Disinfectants. Disinfection is the process of removing microorganisms, including
potentially pathogenic ones, from the surfaces of inanimate objects. The British Standards
Institution further defines disinfection as not necessarily killing all microorganisms,
but reducing them to a level acceptable for a defined purpose, for example a level
which is harmful neither to health nor to the quality of perishable goods. Chemical
disinfectants are capable of different levels of action. The term high level disinfection
indicates destruction of all microorganisms but not necessarily bacterial spores;
intermediate level disinfection indicates destruction of all vegetative bacteria including
Chemical disinfectants, antiseptics and preservatives 201
1 Introduction 3.3.3 Formaldehyde-releasing agents
3.4 Biguanides
2 Factors affecting choice of 3.4.1 Chlorhexidine and alexidine
antimicrobial agent 3.4.2 Polyhexamethylene biguanides
2.1 Properties of the chemical agent 3.5 Halogens
2.2 Microbiological challenge 3.5.1 Chlorine
2.2.1 Vegetative bacteria 3.5.2 Hypochlorites
2.2.2 Mycobacterium tuberculosis 3.5.3 Organic chlorine compounds
2.2.3 Bacterial spores 3.5.4 Chloroform
2.2.4 Fungi 3.5.5 Iodine
2.2.5 Viruses 3.5.6 lodophors
2.2.6 Protozoa 3.6 Heavy metals
2.2.7 Prions 3.6.1 Mercurials
2.3 Intended application 3.7 Hydrogen peroxide and peroxygen
2.4 Environmental factors compounds
2.5 Toxicity of the agent 3.8 Phenols
3.8.1 Phenol (carbolic acid)
3 Types of compound 3.8.2 Tar acids
3.1 Acids and esters 3.8.3 Non-coal tar phenols (chloroxylenol
3.1.1 Benzoic acid and chlorocresol)
3.1.2 Sorbicacid 3.8.4 Bisphenols
3.1.3 Sulphur dioxide, sulphites and 3.9 Surface-active agents
metabisulphites 3.9.1 Cationic surface-active agents
3.1.4 Esters of p-hydroxybenzoic acid 3.10 Other antimicrobials
(parabens) 3.10.1 Diamidines
3.2 Alcohols 3.10.2 Dyes
3.2.1 Alcohols used for disinfection and 3.10.3 Quinoline derivatives
antisepsis 3.11 Antimicrobial combinations
3.2.2 Alcohols as preservatives
3.3 Aldehydes 4 Disinfection policies
3.3.1 Glutaraldehyde
3.3.2 Formaldehyde 5 Further reading
10
Mycobacterium tuberculosis but may exclude some viruses and fungi and have little
or no sporicidal activity; low level disinfection can destroy most vegetative bacteria,
fungi and viruses, but this will not include spores and some of the more resistant
microorganisms. Some high level disinfectants have good sporicidal activity and have
been ascribed the name 'liquid chemical sterilant' or 'chemosterilant' to indicate that
they can effect a complete kill of all microorganisms, as in sterilization.
Antiseptics. Antisepsis is defined as destruction or inhibition of microorganisms on
living tissues having the effect of limiting or preventing the harmful results of infection.
It is not a synomym for disinfection (British Standards Institution). The chemicals
used are applied to skin and mucous membranes, therefore as well as having adequate
antimicrobial activity, they must not be toxic or irritating for skin. Antiseptics are mostly
used to reduce the microbial population on the skin prior to surgery or on the hands to
help prevent spread of infection by this route. Antiseptics are often lower concentrations
of the agents used for disinfection.
Preservatives. These are included in pharmaceutical preparations to prevent microbial
spoilage of the product and to minimize the risk of the consumer acquiring an infection
when the preparation is administered. Preservatives must be able to limit proliferation
of microorganisms that may be introduced unavoidably during manufacture and use of
non-sterile products such as oral and topical medications. In sterile products such as
eye drops and multidose injections, preservatives should kill any microbial contaminants
introduced inadvertently during use. It is essential that a preservative is not toxic
in relation to the intended route of administration of the preserved preparation.
Preservatives therefore tend to be employed at low concentrations and consequently
levels of antimicrobial action also tend to be of a lower order than for disinfectants or
antiseptics. This is illustrated by the requirements of the British Pharmacopoeia (1993)
for preservative efficacy where a degree of bactericidal activity is necessary, although
this should be obtained within a few hours or over several days of microbial challenge
depending on the type of product to be preserved.
Other terms are considered in Chapter 11 (see section 1.1 and Fig. 11.1).
There are around 250 chemical entities that have been identified as active
components of microbiocidal products in the European Union. The aim of this chapter
is to introduce the range of chemicals in common use and to indicate their activities
and applications.
2 Factors affecting choice of antimicrobial agent
Choice of the most appropriate antimicrobial compound for a particular purpose depends
on:
1 properties of the chemical agent;
2 microbiological challenge;
3 intended application;
4 environmental factors;
5 toxicity of the agent.
202 Chapter 10
Properties of the chemical agent
The process of killing or inhibiting the growth of microorganisms using an antimicrobial
agent is basically that of a chemical reaction, and the rate and extent of this reaction
will be influenced by the factors of concentration of chemical, temperature, pH and
formulation. The influence of these factors on activity is discussed fully in Chapter 11
and is referred to in discussing the individual agents. Tissue toxicity influences whether
a chemical can be used as an antiseptic or preservative and this unfortunately limits the
range of chemicals for these applications or necessitates the use of lower concentrations
of the chemical.
Microbiological challenge
The types of microorganism present and the levels of microbial contamination (the
bioburden) both have a significant effect on the outcome of chemical treatment. If the
bioburden is high, long exposure times or higher concentrations of antimicrobial may
be required. Microorganisms vary in their sensitivity to the action of chemical agents.
Some organisms, either because of their resistance to disinfection (for further discussion
see Chapter 13) or because of their significance in cross-infection or nosocomial (hospital
acquired) infections, merit attention.
The efficacy of an antimicrobial agent must be investigated by appropriate capacity,
challenge and in-use tests to ensure that a standard is obtained which is appropriate to
the intended use (Chapter 11). In practice, it is not usually possible to know which
organisms are present on the articles being treated. Thus, it is necessary to categorize
chemicals according to their antimicrobial capabilities and for the user to have an
awareness of what level of antimicrobial action is required in a particular situation
(Table 10.1).
Chemical disinfectants, antiseptics and preservatives 203
Table 10.1 Levels of disinfection attainable
Microorganisms
killed
Microorganisms
surviving
Low
Most vegetative
bacteria
Some viruses
Some fungi
M. tuberculosis
Bacterial spores
HBV and prions
as in Creutzfeldt-
Jakob disease
Disinfection level
Intermediate
Most vegetative
bacteria including
M. tuberculosis
Most viruses
including hepatitis
B virus (HBV)
Most fungi
Bacterial spores
Prions
High
All microorganisms
unless extreme
challenge or
resistance exhibited
Extreme challenge
of resistant bacterial
spores
Prions?
(insufficient data)
2.2.1 Vegetative bacteria
At in-use concentrations, chemicals used for disinfection should be capable of
killing most vegetative bacteria within a reasonable contact period. This includes
'problem' organisms such as Listeria, Campylobacter, Legionella and methicillin-
resistant Staphylococcus aureus (MRSA). Antiseptics and preservatives are also expected
to have a broad spectrum of antimicrobial activity but at the in-use concentrations,
after exerting an initial biocidal effect, their main function may be biostatic. Gram-
negative bacilli, which are the main causes of nosocomial infections, are often more
resistant than Gram-positive species. Pseudomonas aeruginosa, an opportunistic
pathogen (i.e. is pathogenic if the opportunity arises; see also Chapter 1), has gained a
reputation as the most resistant of the Gram-negative organisms. However, problems
mainly arise when a number of additional factors such as heavily soiled articles or
diluted or degraded solutions are involved.
2.2.2 Mycobacterium tuberculosis
Mycobacterium tuberculosis (the tubercle bacillus) and other mycobacteria are resistant
to many bactericides. Resistance is either (a) intrinsic, mainly due to reduced cellular
permeability or (b) acquired, due to mutation or possibly the acquisition of plasmids,
although this has yet to be shown: see also Chapter 13. Tuberculosis remains an important
public health hazard, and indeed the annual number of tuberculosis cases is rising
in many countries. The greatest risk of acquiring infection is from the undiagnosed
patient. Equipment used for respiratory investigations can become contaminated with
mycobacteria if the patient is a carrier of this organism. It is important to be able to
disinfect the equipment to a safe level to prevent transmission of infection to other
patients (Table 10.2). A synergistic mixture of alkyl polyguanides and alkyl quaternaries
has recently been shown to be more effective than glutaraldehyde against M. tuberculosis
and M. avium-intracellulare (see also section 3.11).
2.2.3 Bacterial spores
Bacterial spores are the most resistant of all microbial forms to chemical treatment.
The majority of antimicrobial agents have no useful sporicidal action, with the exception
of the aldehydes, halogens and peroxygen compounds. Such chemicals are sometimes
used as an alternative to physical methods for sterilization of heat sensitive equipment.
In these circumstances, correct usage of the agent is of paramount importance
since safety margins are lower in comparison with physical methods of sterilization
(Chapter 20).
The antibacterial activity of disinfectants and antiseptics is summarized in Table
10.2.
2.2.4 Fungi
The vegetative fungal form is often as sensitive as vegetative bacteria to antimicrobial
agents. Fungal spores (conidia and chlamydospores; see Chapter 2) may be more
204 Chapter 10