It is likely that many factors are involved in the phenomenon of bacterial superinfection. In Table 2 we have summarized potential pathogenic mechanisms that may contribute to bacterial adherence during viral respiratory disease, which we will briefly discuss in the following paragraphs.
4.1Physical damage to respiratory tract epithelium
The intact respiratory epithelium efficiently removes bacteria that have been aspirated into the airways, mainly by ciliary cleaning mechanisms. In primary ciliary dyskinesia, bacterial pulmonary infections are common . Viral infections are thought to induce secondary ciliary impairment. RSV infection induces ciliary injury resulting in ciliostasis, clumping and loss of cilia from live cells in vitro . Influenza virus infection is accompanied by cytological changes in the ciliated columnar epithelium leading to necrosis of the bronchial epithelial lining and in addition to cellular lesions in the alveoli 
It was shown that Pseudomonas aeruginosa adheres to injured tracheal cells of mice during systemic influenza virus infection . In this study no adherence could be found to the intact virus-infected mucosa, basal membrane or to cells recovered from bronchial alveolar lavages. These observations would suggest that apart from specific adherence mechanisms to viral infected cells, there is also an aspecific or ‘opportunistic’ adherence phenomenon which occurs to injured or altered tissues.
In otitis media, impairment of the Eustachian tube function is thought to play an important role in secondary infection. Tube obstruction due to local inflammation leads to an impaired clearance and aeration mechanism. In the chinchilla model, mentioned previously, desufflation of the middle ear in animals inoculated with S. pneumoniae led to an increased incidence of otitis media compared to animals that had not undergone desufflation (45% vs. 21%) . Thus, swelling of the mucosa and functional impairment of the Eustachian tube induced by viral infection may be another mechanism by which viruses promote bacterial superinfection.
4.2Bacterial cellular interaction
Already in 1978 Bartelt et al. showed that trypsin treatment of uninfected cells reduces bacterial adherence, suggesting that a protein receptor is needed for bacterial adherence . Subsequent studies revealed that antiviral antibodies blocked streptococcal adherence to influenza A virus-infected cells . This suggested that viral glycoproteins expressed on the infected host cell membrane could function as receptors for bacteria. During replication of influenza virus, neuraminidase and hemagglutinin are inserted into the host cell membrane. These viral glycoproteins are potential receptors for bacteria. Indeed it was found that streptococcal adherence to influenza A-infected MDCK cells could be blocked by neuraminidase treatment . In addition, hemagglutinin also acted as a receptor by which streptococci adhere during an influenza A virus infection. However, both pretreatments did not inhibit staphylococcal adherence: thus other cellular receptors might be involved in virus-induced increased staphylococcal adherence that are still undefined.
During RSV infection, glycoproteins F and G are inserted into the host cell membrane. In a recent study, glycoprotein G was found to be involved in increased binding of N. meningitidis to RSV-infected epithelial cells .
Apart from glycoproteins, virus infections may induce other changes of the host cell membrane that may contribute to bacterial adherence. Upregulation of pre-existing receptors like CD14, CD15 and CD18 was documented for RSV-infected HEp-2 cells . Both CD14 and CD15 were found to be associated with enhanced adherence of non-piliated N. meningitidis to RSV-infected cells.
In the attachment of NTHI to epithelial cells, fimbriae, known as outer membrane protein P5 homologous fimbriae (P5 fimbriae), play a major role. When pneumocyte type II cells (A549) were preinfected with RSV, P5 fimbria-mediated binding to yet undefined receptors was enhanced, suggesting that RSV infection of these pneumocytes increased the attachment sites for NTHI .
Another upregulated receptor that has been associated with increased bacterial adherence is the platelet activating factor (PAF) receptor. This receptor is found on endothelium and is a receptor for pneumococci only when the cells are activated by cytokines like tumor necrosis factor α and interleukin 1α[39,40]. Both cytokines are produced during RSV infection, but no studies have been conducted yet to address the possible role of the PAF receptor in enhanced pneumococcal adherence as a result of a prior viral infection.
Molecules present in the extracellular matrix provide another mechanism for bacterial binding. They serve as a cross-link between the bacteria and virus-induced changes on host-cell membrane. For example, fibrinogen enhances adherence of group A streptococci (GAS) to influenza A-infected cells . It seems likely that either neuraminidase or hemagglutinin (or both) is (are) responsible for the direct binding of fibrinogen, because fibrinogen-mediated adherence was variable with the subtype of influenza virus used in the assay. Moreover, the authors in this study also showed that tunicamycin, an inhibitor of glycosylation of viral glycoproteins like neuraminidase and hemagglutinin, dramatically decreased fibrinogen-mediated binding of GAS to influenza virus-infected cells.
Finally, Sanford et al. studied the molecular properties of adhesins involved in staphylococcal adherence to influenza A-infected MDCK cells . After heat treatment, it was shown that adhesins mediating binding to virus-infected cells were more heat-labile than adhesins involved in binding to non-infected cells.
Autoclaving and heat treatment, however, were unsuccessful in completely aborting adherence. As adhesins are proteins, it was hypothesized that denaturing surface-exposed adhesins may expose a deeper layer of adhesins, probably of another chemical structure.
In an earlier study of the same group it was shown that two other proteins, clumping factor and protein A, that are present in supernatants of heat-treated staphylococci act as adhesins to normal cells but not to virus-infected cells . Their conclusion was that different subsets of adhesins might play a role in the mechanism of virus-induced bacterial superinfection compared to the adhesins involved in a primary bacterial infection.