Features: Microbes and Emerging Infections: the Compulsion To Become Something New

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 different-and, 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 animal reservoirs.

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.

Figure 1

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 infectious diseases.

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, and elsewhere.

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 penicillin-and 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 Remain Essential

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 substantial harm.

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 life."

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.


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