Gram-Positive Bacilli

The group of Gram-positive bacillary organisms broadly known as “aerobic actinomycetes” consists of heterogeneous and taxonomically divergent genera. They are found in a wide variety of natural and man-made environments but are rarely considered a part of the normal human flora, with infections normally originating from exogenous sources. An extensive number of genera have been described, but only a minority of these has been associated with human or veterinary health. The association with human disease is usually of an opportunistic nature, either through accidental means of inoculation or through involvement with immunocompromising conditions in the host. They cause a wide spectrum of diseases in humans, which may differ greatly between the genera and even between species, but which also may have a great amount of overlap. The occurrence of such infections is probably greater than appreciated, since many may go unrecognized. Etiologic prevalence of specific genera and species varies geographically within the United States and worldwide. Traditional phenotypic identification methods for separation of the many genera and species of aerobic actinomycetes have found great difficulties. Recent use of chemotaxonomic analyses and emerging technologies such as molecular analysis of nucleic acids, and more recently proteomics for identification to the genus/species level, has provided a far more robust technique to understand the organisms’ relatedness, distribution, epidemiology, and pathogenicity in humans.

The majority of the organisms described in this chapter are part of the commensal microbiota associated with the mucocutaneous surfaces of the human and animal digestive tract, being found in the mouth, small and large intestines, urogenital tract, and skin. The various features of non-spore-forming anaerobic gram-positive organism including Propionibacterium, Lactobacillus and Actinomyces are described in the chapter. Biochemical characteristics of Actinomyces and related bacteria encountered in human infection are identified. Far-more-precise identifications can be obtained by 16S rRNA gene sequence analysis. DNA can be rapidly and reliably purified from members of this group by using commercially available kits, such as GenElute (Sigma-Aldrich), and the 16S rRNA gene can be amplified using “universal” primers that amplify all members of the domain bacteria. A recent survey revealed that the method most commonly used for testing anaerobes was the Etest, which has been found to be useful and reliable. Etests should be performed on Brucella blood agar and are optimally read after 48 h, to allow sufficient bacterial growth.

This chapter describes the currently available methodologies and their interpretation for susceptibility testing of anaerobes. At present, alternative testing methods include limited agar dilution, broth microdilution (for the B. fragilis group), and the Etest gradient strip method. Tigecycline, a glycylcycline derivative of minocycline, was recently introduced for treatment of intra-abdominal and skin and soft tissue infections. Several studies have found very good activity against anaerobes, with rare strains exhibiting tigecycline MICs of >8 µg/ml. Extensive taxonomic changes have occurred within the non-spore-forming gram-positive bacilli, especially with those previously in the genus Eubacterium. It is important to compare the drug-containing plates to the drug-free control plate when reading the tests, as different species of anaerobic bacteria can have very differently appearing spots, ranging from mucoid-opaque, as with the B. fragilis group, to gray-transparent, as with Bacteroides ureolyticus. Broth microdilution MIC determinations require criteria similar to those for the agar dilution procedure for reading end points: the concentration at which the most significant reduction in growth is observed and interpreted as the MIC. Current methodologies allow for accurate surveillance or individual isolate testing by most laboratories. Future studies comparing broth microdilution to the reference agar method and the anticipated development of an improved microdilution system will result in better standardization of the more-user-friendly method and, possibly, more widespread commercial availability.

The group of gram-positive bacillary organisms broadly known as “aerobic actinomycetes” consists of a vast array of taxonomically heterogeneous and divergent genera. This chapter deals only with those genera having the most impact on human health care, i.e., those that usually affect primarily patients with immunocompromising conditions. The combination of increasing conditions impairing host resistance to invasion by environmental pathogens and the rapidly progressing technological capability to identify isolates recovered from such patients has allowed a wide expansion of the species known to be capable of causing disease in humans (albeit under special circumstances). A better understanding of the epidemiology, clinical course, and antimicrobial susceptibilities of aerobic actinomycetes is essential. Molecular identification and typing techniques will also bring a better understanding of the interrelatedness of the various genera and species. Evidence-based evaluation is paramount to rapid diagnosis and early choice in therapeutic modalities, aimed to achieve better outcomes in patients. Clinical laboratories must become aware of the role now played by the aerobic actinomycetes in disease and must work with clinical care providers to determine when identification, susceptibility testing, and therapeutic or surgical interventions are necessary.

All bacterial species and phyla are involved in the phenomenon of resistance to antibacterial agents, sometimes posing genuine therapeutic problems. The strategy of Enterobacteriaceae faced with the aggression represented by the oxyimino cephalosporins was the production of conventional enzymes commonly known as extendedbroad-spectrum β-lactamases (ESBLs). The prevalence of nosocomial infections varied from 5 to 17% for patients admitted to ICUs in the hospital setting. In a survey conducted in central Europe, all of the strains of Staphylococcus aureus were susceptible to mupirocin, except in the case of Italy, where 1.5% of methicillin-resistant S. aureus (MRSA) strains were resistant. In S. aureus, the main mechanism of resistance of quinupristin-dalfopristin is methylation of the 23S rRNA with cross-resistance with macrolides, lincosamides, and streptogramin B. In the inducible type of resistance, quinupristin remains active because it is not an inducer of methylase; if constitutive, quinuprisitin is inactive and the combination become bacteriostatic due to alteration of quinupristin-dalfopristin activity in vitro and in vivo. S. pneumoniae is one of the major pathogenic agents of community-acquired or nosocomial parenchymatous respiratory tract infections (RTIs). The frequency of mutation of Mycobacterium tuberculosis varies according to the different antituberculosis agents. Antibiotic resistance in M. tuberculosis has long been known, particularly with streptomycin and isoniazid. Primary resistance of M. tuberculosis to isoniazid is greater than 10% in some countries.

Peptide antibiotics differ structurally from conventional peptides in that they are cyclic and contain ester bonds and D-configured or unusual amino acids, unusual nuclei (such as saturated or unsaturated fatty acids), and heterocycles or amino or nonamino sugars. The peptide antibiotics can be divided into seven groups: I, linear peptides; II, cyclic peptides; III, glycopeptides; IV, lipoglycopeptides; V, lipopeptides; VI, thiazolopeptides; and VII, thiopeptides and chromopeptides. Polypeptide antibiotics induced during an infection in insects, such as cecropins or attacins, have been described in this class. It is highly effective against gram-negative bacilli like Escherichia coli and Pseudomonas aeruginosa, whereas gram-positive cocci, such as Staphylococcus aureus, are poorly inhibited. The majority of peptides of natural or synthetic origin cannot be used as therapeutic agents, as they are unstable in biological liquids, they are toxic, their antibacterial spectrum is limited, and they manifest only weak activity. The cyclic peptides are among the best-studied peptide antibiotics. The majority of molecules have two chlorine atoms on the triphenyl ether nucleus. The teichomycins are lipoglycopeptide antibiotics obtained from fermentation of an actinomycete, Actinoplanes teichomyceticus. In order of inhibitory activity, the pneumocandins may be classified as follows: B0 > A0, A4 > A2, B2 > C0. The bacitracin group consists of hexapeptide antibiotics with a substituted thiazolidine nucleus.