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Microbiology and Laboratory Diagnosis 1
1
Microbiology and Laboratory Diagnosis
Keith Cartwright
1. Introduction
1.1. Historical Background
In 1887, Anton Weichselbaum, a Viennese doctor, was the first to report the
isolation of meningococci from patients with meningitis (1). Shortly after, came
the first description of lumbar puncture in living patients (2), leading to the iso-
lation of meningococci from acute cases of meningitis. Three years later, Kiefer
grew meningococci from the nasopharynx of cases of meningococcal disease,
and from their contacts (3), a finding of immense significance in advancing
understanding of the epidemiology and pathogenesis of the disease. Early
serological typing systems demonstrated that there were important differences
between meningococci in terms of their virulence (4).
1.2. Meningococcal Carriage and Disease
It is believed that meningococci only occur in humans. They have never
been isolated from other animals, possibly owing to their inability to acquire
iron from any other than human sources (transferrin and lactoferrin). Their
fastidious nature makes it most unlikely that there are any important environ-
mental reservoirs. Meningococci form part of the normal commensal flora and
can be isolated from the nasopharynx of approx 10% of individuals overall.
Nasopharyngeal carriage is age-dependent, peaking in late teenage and early
adulthood at 20–30% or more, but with low prevalence in the young and in
the elderly. It is not clear whether acquisition of a new meningococcus in the
nasopharynx results in respiratory illness. Meningococci may also be isolated
from the urethra and from the rectum from time to time and appear to be
capable of causing urethritis.
From:Methods in Molecular Medicine, vol. 67: Meningococcal Disease: Methods and Protocols
Edited by: A. J. Pollard and M. C. J. Maiden © Humana Press Inc., Totowa, NJ
1
2 Cartwright
Invasion is a rare phenomenon, though probably more frequent than would be
suggested by the measured rates of disease. It is well-recognized that a small pro-
portion of young children may present in hospital with a mild febrile illness that
resolves rapidly without antibiotic treatment and from whom a meningococcus is
subsequently isolated from blood cultures. For both ethical and logistic reasons,
blood culture studies of febrile (but otherwise healthy) children in the community
are difficult to mount. Were they to be undertaken with large numbers of partici-
pants, it seems likely that they would identify at least a small number of febrile
children from whose bloodstream a meningococcus could be isolated.
Is it important to confirm the diagnosis in cases of suspected meningococcal
infection? The answer must be in the affirmative, both for the optimal management
of the patient and his or her contacts, and also for the epidemiological added value.
Though meningococci are almost invariably sensitive to penicillin, the exclusion
of other causes of meningitis and septicemia remains a key rationale for the full
microbiological investigation of both these conditions. Without a detailed under-
standing of the range of meningococci causing human disease, and the age groups
affected, development of effective vaccines is impossible.
1.3. The Changing Pattern of Meningococcal Disease Diagnosis
Diagnostic algorithms in suspected meningococcal infection have changed
considerably in the UK over the last 10 years. The drivers have been changes
in clinical management and changes in disease epidemiology, allied to techni-
cal advances in the laboratory.
In the UK and in other countries where most patients with suspected meningo-
coccal disease present first to a primary care medical practitioner, a substantial and
increasing proportion of patients are being treated with a dose of parenteral benzyl-
penicillin. To date, all but one of the published studies (together with unpublished
data) support the efficacy of this early management step. Though beneficial,
administration of benzylpenicillin prior to the patient’s admission to hospital
normally renders blood cultures sterile.
It has been suggested that general practitioners administering benzylpenicil-
lin to patients with suspected meningococcal disease should take blood cul-
tures prior to administering the antibiotic, sending them in to hospital with the
patient. This diagnostic step is theoretically possible, but would present a num-
ber of logistic difficulties. It is probably not warranted now that good
nonculture diagnostic techniques are available (seeSubheading 2.1.).
1.4. Microbiological Investigation as Part of the Early
Management of Meningococcal Infection
There is strong evidence to support the view that delay in the active manage-
ment of meningococcal infection is a major factor increasing the risk of a poor
Microbiology and Laboratory Diagnosis 3
outcome. Studies in various countries have documented some of the reasons
for delay in treatment. One of the most frequent reasons for failing to institute
prompt treatment is the fear that initiation of antibiotic treatment may adversely
affect the microbiological investigations. As a consequence, a patient may
arrive in hospital, be subjected to initial clinical evaluation and may be sus-
pected of having meningococcal meningitis. A lumbar puncture may be
ordered, and antibiotics withheld pending the results of the lumbar puncture. In
a busy pediatric or adult medical unit, this may take an hour or two, or some-
times longer, to arrange. This is unacceptable. As soon as meningococcal
infection is suspected, blood cultures should be drawn, a drip set up, and intra-
venous antibiotics commenced. A lumbar puncture (if deemed appropriate) can
then be carried out at the earliest available opportunity. Because it takes at
least an hour for antibiotics to begin to arrive in the sub-arachnoid space (even
when given by the intravenous route), the chances of isolating a meningococ-
cus (or other bacterium) from the cerebrospinal fluid (CSF) are still high. Even
if CSF cultures are negative, the diagnosis may be confirmed by microscopic
examination of CSF, by latex agglutination tests, or by amplification of micro-
bial DNA by polymerase chain reaction (PCR).
It is also not widely appreciated that meningococcal DNA is cleared only
slowly from the CSF in meningococcal meningitis. If the patient is too unwell
or too unstable for lumbar puncture to be contemplated at the time of admis-
sion to hospital, and if the diagnosis has not been established within the first
24–48 h, a lumbar puncture is still likely to give a positive PCR result even on
d 3 or d 4 of inpatient management. Such a late lumbar puncture will only be
needed rarely, but the possibility should be borne in mind.
1.5. Changing Perceptions of Lumbar Puncture
Lumbar puncture is now used less frequently, especially by pediatricians (5).
This change in clinical practice has arisen from a combination of concern over its
perceived dangers, together with a sense of its lack of contributory value in some
situations. Coning, frequently fatal, may occur in about 1% of cases of meningo-
coccal meningitis where lumbar puncture is undertaken, and lumbar puncture may
exacerbate hemodynamic instability in a patient verging on the brink of shock.
There is also an increasing understanding that analysis of CSF may provide little
additional information relevant to the management of the acutely ill patient (espe-
cially if fever and a vasculitic rash are present and a diagnosis of meningococcal
infection is overwhelmingly likely). Add to this the fact that the results of all initial
examinations (protein, glucose, cell count, and Gram-stained smear) may be nega-
tive and yet a meningococcus may be grown on the following day from 5–10% of
patients(5), and the exercise of caution over the use of lumbar puncture in children
is very understandable.
4 Cartwright
The same is not true in adults with symptoms and signs of meningitis. Here,
the epidemiology of bacterial meningitis is very different (6). A wider range of
pathogens is possible, including the pneumococcus, and other more arcane bac-
teria such as Listeria monocytogenes. A few patients with pneumococcal men-
ingitis may have a vasculitic rash and their infection may be confused on
clinical grounds with meningococcal meningitis or septicemia. The overriding
importance of accurate diagnosis of meningitis in adults is the risk (small as
yet in the UK, but substantial in countries such as Spain, France, and South
Africa) of true penicillin, or penicillin- and cephalosporin-resistant infection.
Lumbar puncture is still the most important investigation in adult patients with
suspected bacterial meningitis (7).
2. Specific Clinical Issues Impacting on Microbiological Diagnosis
2.1. Effect of Early Parenteral Antibiotic Treatment
on Diagnostic Investigations
In the 1980s, the great majority of patients in the UK with suspected menin-
gococcal meningitis were not treated with benzylpenicillin prior to hospital
admission. In such patients (both adults and children), blood cultures were posi-
tive in about 50%, and if meningitis was present and a lumbar puncture was
undertaken, the CSF would either yield Gram-negative diplococci on the
stained smear, or a meningococcus would be isolated on culture in more than
90% of cases.
Alternative diagnostic methods had to be devised to cope with patients with
negative blood cultures, and in whom lumbar puncture was contraindicated.
Throat swabs have proved of great value in this situation, giving a positive
result in up to 50% of patients, a proportion that is largely unaffected by prior
benzylpenicillin treatment (7). Per-oral swabs give a better yield than per-
nasal swabs. If the intention is to isolate a meningococcus, the swab must be
plated out as soon as it is obtained. A swab taken in the middle of the night
cannot be left to be cultured in the morning.
If a skin rash is present, aspiration of an affected area of skin may yield
diplococci on a Giemsa-stained smear, or in a somewhat smaller proportion of
cases, a positive culture. Agglutination of latex particles coated with meningo-
coccal serogroup-specific antibodies by meningococci of the homologous
serogroup can be made more sensitive by inducing better agglutination by
means of ultrasound enhancement.
Demonstration of a rising antimeningococcal antibody titer between acute
and convalescent serum samples may also be helpful for epidemiological rea-
sons, though it does not provide information at the time that it is needed for the
acute management of the patient.
Microbiology and Laboratory Diagnosis 5
However, the test that has emerged from the status of a research tool into
one of fundamental utility is the detection of meningococcal DNA following
its amplification by PCR. In the UK, the Public Health Laboratory Service
(PHLS) Meningococcal Reference Unit (MRU) located at the Manchester
Public Health Laboratory provides this test. From an initial experimental clini-
cal service in 1996, the service has grown such that there were more than 16,000
requests for meningococcal PCR in 1999. The PCR test can be carried out
on peripheral blood or on CSF, and is thought to be specific for meningo-
coccal DNA.
2.2. CSF with Polymorphs but no Organisms Seen or Grown
Another common clinical situation is that in which a febrile child or adult is
subjected to lumbar puncture to exclude the possibility of meningitis, and tur-
bid CSF is obtained, in which neutrophils are observed on microscopy and
from which no bacteria are grown. Though neutrophils may occasionally pre-
dominate in viral meningitis, the presumption is that most such patients have
bacterial meningitis. Most will be treated empirically with a third-generation
cephalosporin such as cefotaxime or ceftriaxone, but the need to establish (if
possible) a more accurate diagnosis lies in the possibility that pneumococci
(with the small attendant risk of treatment failure with either penicillin or with
cephalosporins) may be the cause of the meningitis.
Antigen-detection tests may be of value here and their sensitivity may be
enhanced considerably by the use of ultrasound. Agglutination tests are prob-
ably inherently less sensitive than PCR tests. Meningococcal PCR testing of
CSF is now widely used in the UK and multiplex PCR tests that will detect
DNA from meningococci, pneumococci and from Haemophilus influenzae
type b are now being evaluated.
2.3. Unusual Presentations of Meningococcal Infection
Patients with meningococcal infection may occasionally present with syn-
dromes other than meningitis or septicemia. Urethritis, conjunctivitis, and
pneumonia are all possibilities, as are septic arthritis, endophthalmitis, peri-
carditis, and other infections of deep, normally sterile, tissues. Isolation of a
meningococcus from a normally sterile site is diagnostic, but more difficulty
arises in the interpretation of the significance of a meningococcus isolated from
a superficial site. Clinical and microbiological judgement may be required, but
if there is doubt, and particularly if the meningococcus is present in substantial
numbers, and is well-endowed with capsular polysaccharide, the isolate should
be treated with a high degree of suspicion. For example, primary meningococ-
cal conjunctivitis should be treated aggressively, because there is a high risk of
6 Cartwright
invasive disease if this is not done. In the US in recent years, there has been an
increase in meningãíoccal infections owing to serogroup Y strains, and these
may have a particular predilection for the respiratory tract.
Chronic meningococcemia is now very rare, accounting for about 1% of all
cases. It is normally diagnosed clinically at first. Blood cultures may need to be
repeated frequently before a positive culture is obtained. Meningococcal blood
PCR will probably be positive in the periods immediately after live meningo-
cocci have been cleared from the bloodstream.
2.3. Clusters
When a sporadic case of suspected meningococcal disease occurs, it is, of
course, impossible to say if it will be followed rapidly by another. For this
reason (as well as for clinical reasons), all suspected cases of meningococcal
disease should be investigated as fully as possible.
There is an ever-present risk that a sporadic case of meningococcal infection
may be followed by others in the same family, school, or community. The
public-health management of clusters of cases is made much more difficult
when the diagnosis is uncertain in one or more of the cases. A typical situation
with which public health-medicine specialists have to cope is that in which one
or more suspected but unconfirmed cases in a school or other defined commu-
nity is followed by a confirmed case (or vice versa). Trying to manage the
possible cluster in such circumstances is extremely difficult. If one or more of
the cases has died, pressure from the community for intervention may be
intense, but might not be justified on epidemiological grounds, were good
diagnostic information to be available from all cases.
Having an accurate knowledge of the characteristics of the responsible
strains is of fundamental value in guiding the management. For example, two
or more cases of serogroup C disease occurring within a few days of each other
within a defined small community would warrant consideration of the use of
vaccine in addition to chemoprophylaxis. This would not apply if the cases
were caused by serogroup B strains, or to a mixture of capsular serogroups.
2.4. Postmortem Diagnosis
Because of the aggressive and rapid nature of the infection, some patients
with suspected meningococcal infection will die before, or very shortly after
arrival in hospital, and before there has been a chance to carry out any investi-
gations. It is the author’s experience that a microbiologist is rarely involved in
the investigation of such cases, only getting to hear of them many hours,
or even days later, by which time chances of a positive culture are remote.
Requests for autopsy are often declined by grieving relatives. Blood and/or
CSF PCR tests should be of great value in this situation, though they are as yet
Microbiology and Laboratory Diagnosis 7
formally untested. Aspiration, microscopy, culture, and PCR testing of any
areas of skin rash are also worth considering.
2.5. The Impact of Changing Epidemiology on Diagnosis
When a patient is suspected of having meningitis, and when there is no
microbiological diagnosis, knowledge of the local epidemiology can be of great
help in guiding management of both case and contacts. For example, in the
UK, the introduction of conjugated Hib vaccines in 1992 has almost eliminated
invasive Hib infections in all age groups, and not just in children. The intro-
duction of conjugated meningococcal group C vaccines in November 1999 will
result in a rapid fall in the incidence of meningococcal disease owing to this
serogroup. Consequently, the relative (though not the absolute) risk of a case
of meningitis of unknown etiology proving to be owing to a pneumococcus,
with the attendant possibility of penicillin resistance, will rise.
3. The Future
3.1. Nonculture Detection of Meningococci from Throat Swabs
Though it is believed that most, if not almost all invasive meningococcal
disease follows initial colonization of the upper respiratory tract, meningo-
cocci can only be cultured from throat swabs in about 50% of cases. PCR test-
ing for detection of meningococci in throat swabs is currently under
development at the PHLS Meningococcal Reference Unit. It may prove a use-
ful addition to the available range of diagnostic techniques.
3.2. PCR Tests for Penicillin Resistance
Clinical isolates of meningococci remain sensitive to penicillin, despite a
small decrease in sensitivity in strains submitted to the England and Wales
reference laboratory over the last few years. To date, there have been only a
handful of reports of (cid:96)-lactamase producing meningococci from clinical cases
(and no cases of treatment failure owing to this cause) and none of these strains
has survived for detailed examination today. Nevertheless, the risk of penicil-
lin resistance remains, with the potential for treatment failure. There would be
some value in having available a molecular method for detection of penicillin
resistance, and in particular, the capacity to identify (cid:96)-lactamase producing
strains. Such a test could be carried out in conjunction with screening and
serogroup-specific PCRs.
3.3. DNA Chips
The pace of development of molecular diagnostics makes it seem increas-
ingly likely that DNA chips for the diagnosis of meningococcal disease (and
for a wide range of other meningitis pathogens) will become available within
8 Cartwright
the next few years. As with meningococcal vaccines, their use in developing
countries is likely to be restricted by cost factors.
References
1. Weichselbaum, A. (1887) Ueber die aetiologie der akuten meningitis cerebro-
spinalis.Fortschr. Med.5, 573–583, 620–626.
2. Quincke, H. I. (1893) Ueber meningitis serosa. Samml. Klin. Vort. (Leipzig)67,
655–694.
3. Kiefer, F. (1896) Zur differentialdiagnose des erregers der epidemischen
cerebrospinalmeningitis und der gonorrhoe. Berl. Klin. Woch.33, 628–630.
4. Gordon, M. H. and Murray, E. G. (1915) Identification of the meningococcus.
J. R. Army Med. Corps.25, 411–423.
5. Wylie, P. A. L., Stevens, D. S., Drake III, W., Stuart, J. M., and Cartwright, K.
(1997) Epidemiology and clinical management of meningococcal disease in
Gloucestershire: retrospective population-based study. BMJ315, 774–779.
6. Begg, N., Cartwright, K. A. V., Cohen, J., Kaczmarski, E. B., Innes, J. A., Leen,
C. L. S., et al. (1999) Consensus statement on diagnosis, investigation, treat-
ment and prevention of acute bacterial meningitis in immunocompetent adults.
J. Infect.39, 1–15.
7. Cartwright, K., Reilly, S., White, D., and Stuart, J. (1992) Early treatment with
parenteral penicillin in meningococcal disease. BMJ305, 143–147.
Meningococci from Clinical Specimens 9
2
Isolation, Culture, and Identification
of Meningococci from Clinical Specimens
Per Olcén and Hans Fredlund
1. Introduction
Humans are the only natural reservoir for meningococci. The appropriate
specimens that should be taken for isolation of meningococci are dependent on
the clinical question. The most appropriate specimen and/or laboratory tech-
niques for microbiological diagnosis in an acutely sick patient with suspected
invasive disease like meningitis/septicemia (1) may be quite different from
those required for diagnosis of the cause of a local infection in eye, upper res-
piratory tract, lower respiratory tract, or urogenital tract, or for the study of the
carrier state of healthy persons.
Culture still forms the backbone of diagnosis in spite of major improve-
ments in nonculture diagnostic methods (see Chapters 3–5), the latter being
especially valuable when cultures are “falsely” negative. This can occur for a
number of reasons, most often owing to antibiotic treatment before culture, but
might also be related to transport media and isolation media. Necropsy tissues
and fluids are also particularly difficult (2,3).
Culture is very important because the availability of an isolate growing in the
laboratory will allow species designation, antibiotic-susceptibility testing (see
Chapter 6), and characterization of an isolate for public-health and epidemiological
purposes (seeChapters 8–22). An evident factor of importance is also that almost
every microbiological laboratory can perform cultures for meningococci.
2. Materials (for Diagnostic Sampling Procedures)
In patients with suspected invasive meningococcal disease, it is logical to
take cultures from the suspected primary site of infection (throat/nasophar-
From:Methods in Molecular Medicine, vol. 67: Meningococcal Disease: Methods and Protocols
Edited by: A. J. Pollard and M. C. J. Maiden © Humana Press Inc., Totowa, NJ
9
