Microbiology (Greek: mikros small; bios life), so called because it primarily deals with organisms too small for the naked eye to see, encompasses the study of organisms that cause disease, the host response to infection and ways in which such infection may be prevented. For our purposes the subject can be broadly classified into general, medical and oral microbiology.
Dental students need both a basic understanding of general and medical microbiology, and a detailed knowledge of clinical oral microbiology in order to diagnose oral microbial infections, which are intimately related to the overall treatment plan for their patients. Moreover, the two major oral disorders—caries and periodontal disease—that the dental practitioner is frequently called upon to treat are due to changes in the oral bacterial ecosystem and the constituent oral microbiome, and a grasp of these disease processes is essential for their appropriate management.
The impact of these infections on the health and welfare of the community is simply astonishing. For instance, caries has gained the dubious distinction of being the most prevalent disease, affecting almost half (44%) of the world population in 2010. It has also been estimated, for instance, that caries and periodontal disease are the most costly diseases that the majority of the population has to contend with during their lifetime, and the number of working hours lost due to these infections and the related cost of dental treatment worldwide amount to billions of dollars per annum (e.g., more than 81 billion dollars in the USA in 2006). This is not surprising as it is generally accepted that periodontal disease is the most common affliction of humankind.
The advent of the human immunodeficiency virus (HIV) infection in the early 1980s and the subsequent concerns on cross infection via contaminated blood and instruments have resulted in an increased regimentation of infection control practices in dentistry. Furthermore, many patients are acutely concerned about possible infection transmission in clinical settings because of the intense, and sometimes unwarranted, publicity given to these matters by the media. The dental practitioner should therefore be conversant with all aspects of infection control in the clinical environment, not only to implement infection control measures but also to advise the dental team (dental surgery assistants, dental hygienists and other ancillary personnel) and to allay patients' unfounded fears. For all these and many other reasons, which the student will discover in the text, the discipline of microbiology is intimately woven into the fabric of dentistry and composes a crucial component of the dental curriculum.
It should also be realized that new microbial diseases emerge incessantly (due to reasons given in the following section), and the book you are now reading is a primer for understanding and managing such future scenarios, especially in the context of infection control.
A note on emerging and re-emerging infections
Infectious agents have been adversaries of humans for millennia. Diseases such as plague have wiped out civilizations in ancient times, while humans in turn have won the battle against microbes in more recent times (e.g., eradication of smallpox). Such new diseases are given the terms emerging infections or re-emerging infections (Fig. 1.1), and they are broadly categorized as:
■ new infections: caused by agents such as new influenza virus strains emerging periodically to cause epidemics, and the mosquito-borne Zika virus infection.
■ 'old' infections: known disease entities where the aetiological agents have been recently identified through advances in technology (e.g., Helicobacter pylori causing gastric ulcer disease).
■ re-emergent infections: diseases that have returned with a vengeance due to genetic and structural transformations and attendant increased virulence of the organism (e.g., drug-resistant Mycobacterium tuberculosis with its 'new bag of tricks').
Fig. 1.1 Global prevalence of some emerging and re-emerging diseases. E. coli, Escherichia coli; HIV, human immunodeficiency virus; SARS, severe acute respiratory syndrome; S. aureus, Staphylococcus aureus; vCJD, variant Creutzfeldt—Jakob disease.
The reasons for their emergence are manifold and include:
■ societal events: economic impoverishment (especially in the developing world), war and civil conflicts, as well as mass population migration.
■ health care: new medical devices, organ/tissue transplantation, immunosuppression, antibiotic abuse and contaminated blood and blood products.
■ human behaviour: increasing number of sexual partners, injectable drug abuse.
■ environmental changes: deforestation, drought, floods and global warming.
■ microbial adaptation: emergence of new species from the wild (e.g., HIV), changes in virulence and toxin production and development of drug resistance.
About this book
This text is divided into six parts in order to highlight the different features of microbiology related to dentistry, but it should be noted that such division is artificial and is merely an attempt to simplify the learning process.
The first few chapters in Part 1 essentially describe general microbiological features of bacteria and viruses and how they cause human infections (i.e., pathogenesis). Diagnostic microbiology, by which clinical microbiologists ascertain the nature of agents causing various infections, is described in Chapter 6. The laboratory aspect of this fascinating subject is analogous to the work of a crime detection bureau! When a specimen (e.g., pus, urine) from a patient with an infectious disease is sent to the laboratory for identification of the offending agent, the clinical microbiologist utilizes many methods and techniques, as well as a fair amount of thought and contemplation, to identify the pathogen/s lurking in the clinical sample. In many situations the pathogen may be dead, in which case other, indirect clues via molecular techniques need to be pursued to incriminate the suspect pathogen. Once an offending pathogen is identified, antimicrobial chemotherapy is the mainstay of treatment; a description of chemotherapeutic agents and how they are chosen in the laboratory is given in Chapter 7.
The host responds to infection by mounting an immune response. A highly abbreviated account of basic immunology is given in Part 2; supplemental reading is essential to augment this material, and the reader is referred to the lists of recommended texts for this purpose. Immunological nomenclature is complex and often difficult: a glossary of terms and abbreviations is therefore provided as an appendix.
Although there are thousands of offending pathogens, only some are of direct relevance to dental practice and to the comprehension of the mechanisms of disease; these are described in Part 3. Arguably this section may appear to be the most daunting part of the book because of the complex nomenclature of microbes; hence only the salient bacterial genera—some of which are more closely related to dental practice (e.g., streptococci) than others (e.g., legionellae)—are outlined. Similarly, the chapters on viruses and fungi are relatively brief, with thumbnail sketches of only the most relevant organisms.
The major infections of each organ system are discussed in Part 4, with emphasis on those that are most relevant to dentistry. The student is strongly advised to cross-refer to organisms and their characteristics (described in Part 3) when studying this section, as the microbial attributes and the diseases they cause form a single continuum.
Part 5 specifically outlines the microbial interactions in the craniofacial region, in both health and disease. This section should be particularly useful for the later years of the dental curriculum, to reinforce the studies in conservative dentistry, periodontics, oral and maxillofacial surgery and oral medicine.
Last but not least, the subject of cross infection and its control in dentistry is encapsulated in Part 6. It provides a comprehensive summary of the routine infection control regimens that must be implemented in every dental practice. The relevance of this information in routine clinical practice cannot be overemphasized, and a thorough understanding of this material should pay rich dividends in years to come.
As the student will discover, the comprehensive nature of this text has made almost all the materials significant. Thus the reader will be intellectually challenged to learn a new concept or terminology in almost every sentence or phrase. In addition, an attempt has been made to summarize the information as key facts, to serve as an aide-mémoire, at the end of each chapter. It is important, however, that the subject matter is augmented with additional reading, and it is to this end that the list of recommended texts is given. The self-assessment quiz at the end of each chapter, which may not cover all aspects of the preceding narrative, should help the reader to assess knowledge assimilation in key areas.
Finally, in most chapters the text is arranged under the following important features of microbiology, which the student must understand in order to deal with infectious diseases:
■ Epidemiology: spread, distribution and prevalence of infection in the community.
■ Pathogenesis: the means by which microbes cause disease in humans, an understanding of which is critical for the successful diagnosis and management of infections.
■ Diagnosis: detection of an infection; this depends on the collection of the correct specimen in the most appropriate manner, and subsequent interpretation of the laboratory results.
■ Treatment: antibacterial, antifungal or antiviral therapy combined with supportive therapy leads to resolution of most infections.
■ Prevention (prophylaxis): immunization is the most useful mode of preventing diseases such as tetanus and hepatitis B; however, increasing public awareness of diseases and their modes of spread significantly helps to curb the spread of infections in the community (e.g., HIV infection).
Further reading
Beikler, T., & Flemming, T. F. (2011). Oral biofilm-associated diseases: trends and implications for quality of life, systemic health and expenditures. Periodontology 2000, 55, 87-103.
Bonita, R., Beaglehole, R., & Kjellstrom, T. (2006). Communicable diseases: epidemiology surveillance and response (Chapter 7). In Basic epidemiology (2nd ed., pp. 117-132). Geneva: World Health Organization.
Brooks, J. F., Carroll, K. C., Butel, J. S., et al. (Eds.), (2013). The science of microbiology (Chapter 1). In Jawetz, Melnick & Adelberg's Medical microbiology (26th ed., pp. 1-8). New York: McGraw Hill. e-Book. Morse, S. S. (1995). Factors in the emergence of infectious diseases.
Emerging Infectious Diseases, 1, 7-15.