Chapter 4 Biofilms in Health and Medicine
Section 8 The Flora of the Major Body Systems
Page 5 The Respiratory Apparatus

The flora of the major body systems

The Respiratory Apparatus

The human pulmonary apparatus consisting of the nasopharynx, the trachea, the bronchioles and the lungs is another of those systems, in which the proximal organs, the lungs are sterile under normal conditions, while in the distal portions the nose and the mouth are among the most heavily colonized regions of the body. 


The protective mechanisms of the respiratory tract

The mechanisms that enable the body to accomplish this remarkable feat are the mucocilliaryesclator, the phagocytic cells (neutrophiles and macrophages) and antibodies, products of the acquired immune system.  Failure of any of these protective mechanisms results in severe, commonly lethal disease.  The importance of these mechanisms in mammals is attested to by the ability of the lungs to tolerate and clear as many as 1 x 105 aspirated cells within 20 minutes (Morc et al. 1009)-(in Wilson).

Most inhaled particles are filtered out in the nose and nasal conchaeor turbinates,which provide a large, mucus covered surface.  As many as 70-80% of the particles entering the airway go no further.  Smaller particles, however, less than 1 µm in diameter, are not removed in the nose, these are instead removed by a mechanism called the mucocilliaryescalator.

The mucocilliary escalator is a remarkable particle entrapment and clearance mechanism which extends from the posterior portion of the nasal cavity through the trachea andnasopharynx and the air conducting bronchioles  out to the terminal bronchioles.  The mechanism consists of ciliated epithelial cells lining the respiratory tract plus a variety of secretorycells whichproduce a watery pericilliary fluid (serous cells) which lies close to the epithelium and a more viscous mucus (goblet cells) which overlies the more fluid layer.  Between these two layers is a thin layer of a surfactant, itself the secretory product of still other epithelial cells (Clara cells).  This complex layer is arranged in such a way that the viscous mucus can float upon and move over the surface of the lower layer of pericilliary fluid.  The movement is created by the enormous number of cilia associated with the ciliated epithelial cells which as they beat move the mucus layer upward, away from the deepest portions of the lungs and to the back of the through where the mucus is either swallowed or expectorated.

In practice, as particles including spores and bacteria enter the airway during inhalation, they impinge on the mucus layer covering the respiratory epithelium.  The cilia beating at a rate of several hundred excursions per minute move the mucus layer along at a rate of 5-20 mm per minute (Wilson).

Given the effectiveness of this mechanism, is seems surprising that any organisms can gain a permanent foothold.  A number of the species inhabiting the human respiratory tract, including important opportunistic pathogens such as Haemophilusinfluenzae, Strep.pneumoniae, Staph. aureus and Strep pyogenes, produce adhesins.  These adhesins enable bacteria to bind to the mucus layer covering the respiratory tract, but why are these not rapidly cleared?  It may be that these organisms are able to alter the mucocilliary escalator so as to prevent its normal function (Wilson).

The flora of the nasoparynx, trachea and bronchi

The normal resident flora of the nasopharynx are dominated by Gram positive organisms such as the α-hemolytic streptococci (viridans group, and occasionally S. pneumoniae), the coagulase positive and negative staphylococci (S. aureus and S. eidermidis) many of which are opportunistic pathogens in compromised host individuals.  In addition to these there are many Gram negative organisms that inhabit the nasopharynx, these include species of Haemophilus, Neisseria and Moraxella, many of which are also potential pathogens.  Another group of organisms found growing on the mucosal surface the respiratory tract are the mollicutes, bacteria lacking cell walls and exhibiting very exacting nutritional requirements of their environment.  These include the genera Ueraplasma and Mycoplasma which are very small organisms (0.2 µm) and possess very small DNA molecules (< 600 kilo bases). Some of these unusual organisms (e.g. Mycoplasmafermentans) are known to be opportunistic pathogens.

Otitis media and the problem of non-culturable specimens

Otitis media and otitis media with effusion are the most common illnesses which cause parents to seek medical care for their children.  It is also the most common  condition for which children require antibiotics and surgery, typically tympanostomy tube placement to permit drainage of effusion from the middle ear.  Curiously, although these bouts of middle ear ache generally resolve, at least temporarily, with antibiotics the presumed organism responsible for the infection is often not culturable from the effusion isolated from the ear at the time of surgery.  Nearly 75% of OM cases are non-culturable, that is, the major organisms associated with this disease Haemophilusinfluenzae, Moraxellacatarrhalis and Streptococcus pneumoniae are usually not recovered when samples are spread on nutrient media designed to grow these organisms.  Does this imply that many cases of OM and OME are due to some other cause?

In a recent investigation (Hall-Stoodley et al. 2006) small specimens of the middle ear mucosa were excised during the insertion of tympanostomy tubes, control samples from OM negative subjects undergoing cochlear implant surgery were also obtained.  These specimens were stained with fluorescent dyes and immuno-fluorescent stains for FISH (Fluorescent In-Vitro Hypridization) for examination under Confocal scanning laser microscopy (CSLM). 

Only 5 of 27 (19%) effusion samples from the subject population were culturable for a pathogen, but 46 of 50 OM samples (92%) observed by CSLM with fluorescent stains or with pathogen specific FISH techniques were positive for the presence of biofilm.

This investigation reveals one of the very important attributes of biofilms, often the microorganisms associated with biofilms fail to grow on media designed to culture the organisms suspected to be responsible for the infection. Despite their failure to grow on media, these organisms are commonly present and capable of growing and causing pathology in situ.