Microbes and Emerging Infections: the Compulsion To
Become Something New
Microbiologists are advised to respect Koch's postulates while
surveying for emerging and reemerging agents of infectious disease
Richard M. Krause
Many factors such as changes in demography, lifestyle, and
agriculture contribute to emerging infectious diseases. And yet, amid
those human-level factors, bacteria are not idle bystanders, waiting for
new opportunities to exploit. Rather, they possess an innate compulsion
to become something new and are constantly evolving. Moreover, the
waxing and waning of epidemics is a biological expression of periodic
shifts in host-microbe associations. For example, the scarlet fever
pandemic of the 19th century provides a case study of such behavior and
may also help to explain the recent increase in incidence of
streptococcal toxic shock syndrome (STSS) and necrotizing fasciitis.
Emerging Bacterial Infections: Genetic Aspects
Emergent bacteria and the diseases they cause may occur as a
consequence of selective evolutionary pressures acting at the genetic
level. Strange as it may seem, ideas suggesting that evolution applies
to microorganisms and that genetics could contribute to the emergence of
new diseases are relatively new. When in medical school during the
mid-1940s, I was taught "The application to bacteria of terms that
have been coined to express changes in form or function occurring in
higher plants or animals is not without its dangers, and it is possible
that there is little real justification for the use of such a term as
mutation, in connection with the variations which bacteria may
undergo" (from the third edition of a widely used medical school
textbook, Topley and Wilson, published in 1946).
Such caution is not surprising for medical science in 1946. The
mutational origin of bacterial variants had been demonstrated only a few
years earlier, in 1943. Although Burnet perceived the occurrence of
bacterial mutations in a 1936 paper, Induced Lysogenicity and
Mutation of Bacteriophage within Lysogenic Bacteria, his work had
little impact at the time.
We now realize that versatile features of bacterial genetics enhance
the ability of microbes to overcome their natural ecological
constraints, including species barriers or medical barriers, such as
antibiotics and vaccines, and to emerge as something differentand,
not uncommonly, with enhanced virulence. For instance, toxigenic Escherichia
coli, which can cause bloody diarrhea and the hemolytic uremic
syndrome, and group A streptococci (GAS), which can cause toxic shock
syndrome (STSS), very likely have acquired new vigor as a result of
genetic events and evolutionary selection. The new agents then exploit
changing circumstances of the ecosystem that are brought about by
perturbations in nature and human behavior.
The historical record reveals that major "new" epidemics
ebb and flow only to recur in a new disguise, to become embedded in the
population as an endemic disease, or to disappear. AIDS likely will
follow one of these alternatives in this millennium. Recent research on
the rise and decline of epidemics and pandemics embraces insights that
derive from studying the population biology of host-microbe
associations. For example, there has been speculation, in some cases
supported by evolutionary genetic studies, that tuberculosis and plague
were introduced into human populations in ancient times from domestic
A Case Study: Scarlet Fever
The history of scarlet fever is a good case for examining some of the
factors that can influence the rise and fall of epidemics. Scarlet fever
was very common and one of the most deadly childhood diseases in the
19th century. And yet, even before the antibiotic era, the incidence of
scarlet fever had been declining for decades, and there was a parallel
decline in severity.
Consider the history of scarlet fever in the 19th century. In
general, registries of illness and death were well kept in large cities
in both Europe and the United States, and they provide reliable data on
the incidence of, and mortality from, scarlet fever during this time.
Alan Katz of the University of Hawaii and David Morens of NIAID have
noted that scarlet fever most likely occurred for centuries either as an
endemic disease or as localized epidemics. And then, in the early part
of the 19th century, a pandemic of often fatal scarlet fever appeared
suddenly and swept through Asia, Europe, and the United States (Fig. 1).
Physicians in 1830, reflecting on their past experience, noted a
striking increase in mortality not seen previously, and fatality rates
of up to 30% were often reported. Scarlet fever became the most common
fatal infectious childhood disease, more fatal than measles, diphtheria,
or pertussis, a fact that is difficult to comprehend today.
From 1830 to 1880, pandemic scarlet fever waxed and waned in
incidence and severity. Mortality from the incidence began to decline
about 1880, and by 1930, clinicians in general remarked on this decline,
as well as a gradual fall in the severity of the clinical
manifestations. These changes arose even before the use of antibiotics.
Today, scarlet fever is rare and, when it occurs, the disease is usually
not life threatening, even though pharyngitis persists as a very common
infection of childhood.
Why does streptococcal pharyngitis remain a common infection today,
whereas scarlet fever has all but disappeared? Did the earlier
streptococci associated with scarlet fever possess especially virulent
characteristics that are lacking in the streptococci isolated from
pharyngitis patients today? Did they produce excessive amounts of
scarletina toxin(s) or some other toxin(s)? Satisfactory answers to
these questions are not available, but clues may be forthcoming from
current intensive investigations of the bacterial genetics and
population and evolutionary epidemiology of GAS.
While the role of the genetic evolution of GAS in the occurrence of
the 19th century pandemic of scarlet fever is still a matter of
speculation, there is no doubt about the influence of multiple social
and demographic factors. Foremost among these would have been the
intense crowding in the large industrial cities of England, Europe, and
the United States. I believe that another factor, often overlooked,
played an important role in the spread of pandemic scarlet fever.
This factor involves the explosive progress in transportation during
the 19th century that facilitated a far more rapid spread of
streptococcal disease than before. As horse carriages and sailing ships
gave way to railroads and steamships, millions of travelers, immigrants,
soldiers, and sailors traveled more rapidly and farther in less time
than ever before. Streptococcal disease, cholera, and other major
infections could circle the globe in weeks instead of months and years.
With massive armies traversing continents, the Napoleonic Wars, the
Franco-Prussian War, and the U.S. Civil War undoubtedly enhanced the
spread of streptococcal infections, and this occurred also as recently
as World War II.
In sum, scarlet fever became pandemic because of a virulent
streptococcal clone(s) arising through gene mutations and possibly also
gene transfer; an increase in population density, crowding, slums, and
poor nutrition; and new modes of transportation that ushered in rapid
long-distance travel for large populations of civilians and soldiers.
The incidence and mortality of scarlet fever subsequently declined
because of a loss of streptococcal virulence; population immunity;
sanitation and public health measures; improved housing, nutrition, and
medical care; and widespread use of antibiotics. All these factors and
any combination of them that relate to the scarlet fever pandemic are
generic and can foster the emergence, reemergence, or decline of other
Group A Streptococcus: Links With the Past?
It is tantalizing to speculate that the GAS that caused the lethal
pandemic of scarlet fever a century ago is related to the streptococci
that cause streptococcal toxic shock syndrome, or STSS. Clinicians
generally agree that STSS has become more frequent in recent years, with
outbreaks in Canada, Europe, and the United States, and scattered cases
in Hong Kong, Japan, and elsewhere in the Far East.
STSS and necrotizing fasciitis often begin with a local infection,
often a minor puncture wound, that rapidly produces an extensive
necrotic lesion and might be followed by multiple organ system failure,
toxic shock, and death. Even with aggressive antibiotic therapy, the
death rate from STSS is 15 to 30%, and survivors may be permanently
crippled following amputation for extensive and irreversible tissue
necrosis. Because treatment with antibiotics alone cannot prevent the
cascade of toxic events initiated by streptococcal toxins that occur in
STSS, several of which are superantigens, treatment must be directed
toward neutralizing the toxins and preventing adverse side effects.
Researchers are intensively investigating the genetic and pathogenic
properties of GAS that cause STSS. The findings of this research,
conducted within the context of population and evolutionary biology, are
intriguing. For example, the allelic variation of several genes encoding
putative virulence factors (scarlet fever toxin, M protein, and other
genetic markers) correlates with the increased frequency and severity of
STSS. Moreover, distinct subclones expressing serotypes M1 and M3 of the
M protein may be responsible, in part, for recent increases in episodes
of invasive disease in infected patients in Europe, the United States,
Evidence for a connection to the scarlet fever pandemic of the 19th
century may emerge from the increasing body of research on the genealogy
of GAS, particularly from comparisons of streptococci isolated from
current STSS patients with those from cultures isolated several decades
ago from patients with scarlet fever. Whether one or more of the toxins
implicated in the pathogenesis of STSS are also a property of the
streptococci that caused fatal scarlet fever in the 19th century is not
yet known, but this question may be answered if new methods can be
developed to determine the genome of GAS in formalin-preserved tissues
from patients who died of scarlet fever in the past century.
Host immune responses also suggest a relationship between scarlet
fever and STSS. Current efforts to treat STSS include use of intravenous
immunoglobulin (IVIG), which is a good source of antibodies to numerous
streptococcal toxins, in addition to antibiotics and surgical
debridement. Recently, Rupert Kaul of Mount Sinai and Princess Margaret
Hospitals and associates reported treatment of 21 patients with IVIG (2
g/kg). Survival was 67% versus 34% for 32 historical controls.
Such a favorable outcome with IVIG therapy for the treatment of STSS
has a historical precedent in the use of antitoxin serum in patients
with scarlet fever prior to the antibiotic era. The goal was to
neutralize the scarlatinal toxin(s) and minimize the severe rash and
toxemia from which patients died. Several reports from that era document
that this serum markedly reduced the death rate from scarlet fever.
Also, earlier in the 20th century, bacteriologists sought to prevent
scarlet fever via active immunization with a toxoid of the scarletina
toxin(s). The vaccine reduced or prevented manifestations of scarlet
fever due to subsequent streptococcal infections, including toxemia, and
dramatically reduced the death rate from scarlet fever.
Need for a Vaccine To Prevent Streptococcal Infections
Development of a vaccine to prevent streptococcal infections is under
way in the United States, supported by NIAID and industry, and in
Australia and Europe. In the past, a number of technical barriers have
prevented the development of a streptococcal vaccine. These include the
occurrence of multiple M protein serotypes as the cause of infection,
and type-specific immunity to the M protein. For these reasons, only a
multivalent vaccine would be successful. Finally, in all earlier
studies, the M protein preparations elicited feeble immune responses.
These and other technical problems are currently being circumvented with
the application of recombinant DNA technology and the molecular design
of synthetic antigens based on the known amino acid sequence of the M
protein. There has also been a renewed interest in the search for a
streptococcal antigen(s) that would give broad immunity to the major M
protein serotypes of GAS.
There are four compelling reasons for developing a vaccine to prevent
GAS infections. First, widespread use of such a vaccine would prevent
complications of acute rheumatic fever (ARF) and rheumatic heart disease
(RHD), which continue to occur frequently in nonindustrialized
countries. Second, GAS are slowly, but surely, developing resistance to
more than one antibiotic. Third, although unlikely, GAS associated with
STSS could become more communicable due to genetic evolution and
selection, leading to a higher incidence of STSS. Fourth, there is the
possibility of penicillin resistance. Fortunately, GAS have not yet
developed resistance to penicillinand they may not, since penicillin
has been used widely for 50 years. But, if GAS were to become resistant
to penicillin, no second line of defense exists that is as effective as
penicillin for adequately treating streptococcal sore throats. Without
this defense, primary and secondary prevention of ARF and RHD would be
crippled. Indeed, long-acting penicillin has become a surrogate vaccine
for prevention of group A streptococcal infections.
Emerging Diseases: Surveillance, Analysis by Koch's Postulates
Meanwhile, surveillance efforts in the United States and elsewhere
should be expanded to ensure that emergence and reemergence of other
infectious diseases can be detected promptly wherever they arise.
However, as a public health defense measure, surveillance is a slender
reed that can bend in the storm. Research also is needed to create
strong countermeasures to such diseases, before emergent outbreaks cause
This research effort must embrace a broad array of biological
disciplines in interdisciplinary efforts. The survival of microbes,
vectors, and intermediate hosts and their adaptation to new habitats
need to be better understood. In recent years, Roy Anderson and Robert
May of Oxford University and others have broadened their research on
epidemics to include analysis of population biology as it relates to the
dynamics of disease transmission and the evolution of infectious
diseases. Recommendations to improve vaccination strategies to minimize
persistence of highly contagious diseases, such as measles, is a
practical spin-off of this theoretical work. In addition, the genetic
makeup of microbes and their ability to cause disease must be more fully
understood, including the mechanisms of pathogenesis and the
immunological processes that are mobilized by the body to fight
microbial invasion and infection.
Amid these efforts, we should not discard Koch's so-called
"postulates," as some young scientists suggest. For instance,
one molecular biologist is quoted as saying, "if you play the game
that you have to fulfill Koch's postulates, you fall into a line of
thinking that has been obsolete for decades."
I disagree. Robert Koch reported his discovery of the tubercle
bacillus as the cause of tuberculosis on 24 March 1882, at the monthly
meeting of the Physiological Society of Berlin. Koch's paper was
entitled, simply, Ueber Tuberculose. The logic of his
presentation was so compelling that the audience sat in stunned silence
when he finished speaking and then rose, each, in turn, to shake his
hand. The news electrified the world. Paul Ehrlich later recalled the
evening as the "most important experience of my scientific
Immediately after Koch gave his 1882 lecture on tuberculosis,
Ehrlich, a young colleague who was a whiz at the complexities of
histological staining of tissue sections, improved Koch's staining
procedure in a matter of weeks and described the acid-fast property of Mycobacterium
tuberculosis. This technique is still used, with modification, to
identify tuberculosis in sputum and tissue sections. Acid-fast staining
was the PCR of that time.
Koch's "postulates" are a formalization of the scientific
evidence needed to establish a cause-and-effect relationship between a
microbe and a disease. In the original German, the
"postulates" read as follows:
Wenn es sich nun aber nachweisen liess: erstens, dass der Parasit
in jedem einzelnen falle der betreffenden Krankheit anzutreffen ist
and zwar unter Verhaltnissen, welche den pathologischen Veranderungen
and dem klinischen Verlauf der Krankheit entsprechen; zweitens, dass
er bei keiner anderen Krankheit als zufalliger and nicht pathogener
Schmarotzer vorkommt; and drittens, dass er von dem Korper, vollkommen
isolirt and in Reinculturen hinreichend oft umgezuchtet, im Stande ist,
von Neuem die Krankheit zu erzeugen; dann konnte er nicht mehr
zufalliges Accidens der Krankheit sein, sondern es liess sick in
diesem falle kein anderes Verhaltniss mehr zwischen Parasit und
Krankheit denken, als dass der Parasit die Ursache der Krankheit ist."
In fact Koch did not use the word "postulate," as noted in
the following translation:
To demonstrate or prove, that: first, the parasite is found in every
single case of the certain disease under conditions which correlate to
pathological changes and clinical development; second, the parasite is
not found in other diseases more accidentally and is there
nonpathogenically; and third, if it is able to induce the disease in a
new body after isolation and clean cultivation from a suffered body;
then, it cannot be a random coincidence with the disease, but there is
no other explanation for the relation between parasite and disease than
that the parasite is the cause of the disease.
Today, proof that a specific microbe is the cause of a disease may
not require an animal model or transfer of the disease to another
person. New techniques include tissue cultures or DNA probes. In Koch's
time, however, the concept of "germs" causing disease was
novel, and suspect organisms were injected into animals or persons to
prove causality. Most convincing to those who attended Koch's lecture in
1882 was his demonstration that M. tuberculosis, isolated and
cultured from humans with tuberculosis, produces the disease in animals.
Had he not done so, his report would have been greeted with great doubt
and, perhaps, rejected.
Koch demanded rigorous proof that a particular microbe causes a
specific disease. His 1882 lecture was, and continues to be, a model for
rigorous proof, an essential aspect of scientific inquiry.
Microbiologists who provide anything less when describing emerging and
reemerging infectious diseases typically come to rue the day. In his
time, Koch's thinking was very current, and he would be equally current
today if he were faced with Legionnaires' disease, Lyme disease, or
Ebola virus disease. My advice to the scientists who dismiss Koch and
his accomplishments is that they return to his writings and review how
novel his ideas were within the context of his time.
Current research on the rise and decline of epidemics is broadly
based and includes evolutionary and population genetics of host-microbe
relationships. Within this context, the strains responsible for the
19th-century pandemic of scarlet fever may well have shared virulence
factors with the GAS which currently cause STSS. The strategy to
confront emerging infectious diseases should be the study of infectious
diseases from all points of view. They remain one of the greatest
threats to our society.
Anderson, R. M. 1998.
Analytical therapy of epidemics, p. 23-50. In R. M. Krause (ed.),
Emerging infections. Academic Press, New York.
Brock, T. D. 1998. Robert
Kocha life in medicine and bacteriology, p. 129. ASM Press,
Washington, D.C. Quoted from B. Mollers, Robert Koch. Personlichkeit and
lebenswerk, 1843-1910. Hannover: Schmorl & Von Seefeld; 1950. p.
Katz, A. R., and D. M. Morens. 1992.
Severe streptococcal infections in historical perspective. Clin. Infect.
Kaul, R., A. McGeer, M. K. Norrby-Teglund, B.
Schwartz, K. O'Rourke, J. Talbot, D. E. Low, and The Canadian
Streptococcal Study Group. 1999. Intravenous
immunoglobulin therapy for streptococcal toxic shock syndrome-a
comparative observational study. Clin. Infect. Dis. 28:800-807.
Krause, R. M. (ed.). 1998.
Emerging infections. Academic Press, New York.
Krause, R. M. 1984. Koch's
postulates and the search for the AIDS agent. Rev. Infect. Dis. 6:270-279.
Lederberg, J. 1998.
Infectious agents, hosts in constant flux. ASM News 64:18-22.
Low, D. E., B. Schwartz, and A. McGeer.
1998. The reemergence of severe group A streptococcal disease: an
evolutionary perspective, p. 93-123. In W. M. Scheld, D. Armstrong, and
J. M. Hughes (ed.), Emerging
infections. ASM Press, Washington, D.C.
Musser, J. M., and R. M. Krause.
1998. The revival of group A streptococcal diseases, with a commentary
on staphylococcal toxic shock syndrome, p. 185-218. In R. M. Krause
(ed.), Emerging infections. Academic Press, New York.