Oral and respiratory tract pathogens represent a significant threat to human health. Nosocomial infections are widespread, especially among more vulnerable patients, and are important contributors to morbidity and mortality. A growing number of bacterial infections are becoming harder, and sometimes impossible, to treat as antibiotics become less effective, and vaccination against respiratory viruses either does not exist or has incomplete coverage. When there is an emerging infectious disease outbreak, practicing appropriate hygiene is recommended for both healthcare workers and individuals to limit the spread of infection by breaking the transmission. Oral hygiene could further improve the success rate of hygiene measures, especially against respiratory pathogens.
The data from this in vitro study demonstrated rapid bactericidal and virucidal activity of PVP-I gargle/mouthwash against all respiratory pathogens tested according to European standard requirements. The minimum 15-s contact time proved to be sufficient for PVP-I 7% gargle/mouthwash to be effective at the recommended dilution in Japan of 1:30 (equivalent to a concentration of 0.23% of the active ingredient). PVP-I solution has shown similar rapid antimicrobial activity in previous in vitro studies. Against bacteria, Shimizu et al. demonstrated complete efficacy of PVP-I 0.2% solution against clinical isolates of Klebsiella pneumoniae, Serratia marcescens, Pseudomonas aeruginosa, Alcaligenes faecalis and Alcaligenes xylosoxydans within 30 s using a simple methodology (the broth turbidity method) . In another study, low concentrations of PVP-I gargle/mouthwash (0.23–0.47%) killed methicillin-resistant Staphylococcus aureus (MRSA) and Pseudomonas aeruginosa, including multidrug-resistant strains, within 15-60 s in the presence of oral organic matter from healthy volunteers, while 0.02% benzethonium chloride (BEC) and 0.002% chlorhexidine gluconate (CHG) were ineffective . In a study comparing the bactericidal activities of three gargle/mouthwashes against isolate and standard strains of gram-positive (MRSA) and -negative bacteria (Pseudomonas aeruginosa and Klebsiella pneumoniae), PVP-I (diluted 15-, 30- and 60-fold) elicited rapid killing of all three strains after 30 s of exposure, while cetylpyridinium chloride (CPC) was effective only against gram-negative strains after 60 s of exposure, and CHG was ineffective .
In previous virucidal studies, PVP-I gargle was found to inactivate a panel of viruses including adenovirus, mumps, rotavirus, poliovirus (types 1 and 3), coxsackie virus, rhinovirus, herpes simplex virus, rubella, measles, influenza and human immunodeficiency virus, while CHG, benzalkonium chloride (BAC), BEC and alkyldiaminoethyl-glycine hydrochloride (AEG) gargles were ineffective against adenovirus, poliovirus and rhinovirus . Eggers et al. demonstrated the virucidal activity of PVP-I 4% skin cleanser, 7.5% surgical scrub, 10% solution and 3.2% PVP-I/alcohol solution against Ebola virus and MVA and of PVP-I 7.5% surgical scrub, 4% skin cleanser and 1% gargle/mouthwash against MERS-CoV and MVA within 15 s of application [24, 25]. Application of PVP-I products with concentrations of 0.23–1% for 1–2 min reduced SARS-CoV virus infectivity from 1.17 × 106 TCID50/ml to below detectable levels in a study by Kariwa et al., although shorter contact times were not investigated . Ito et al. reported a reduction in viral infectious titres of avian influenza A viruses (H5N1, H5N3, H7N7 and H9N2) to below detectable limits by incubation for only 10 s with six different PVP-I products including 0.23% gargle and 0.23% throat spray . The anti-influenza activity of PVP-I involves inhibition of viral haemagglutinin binding activity and viral neuraminidase catalytic hydrolysis . In the present study, PVP-I oral solution at a concentration of 0.23% was also effective against non-enveloped rotavirus without interfering substance after 15 s of exposure, which is in contrast to in vitro work by Steinmann et al. , in which 7.5% PVP-I handwash was not active against the non-enveloped viruses tested, and by Sauerbrei and Wutzler , in which PVP-I took 5 min to inactivate polyomavirus SV40 and adenovirus.
In this study, the virucidal and bactericidal activity of PVP-I gargle/mouthwash was evaluated within a short exposure time (15 s) to reflect a similar or shorter time than the actual gargling time in real-life conditions, since the length of time that individuals are willing to keep an antiseptic product in the oral cavity is limited. PVP-I oral solution at a concentration of 0.23% was effective against all pathogens tested in this study after the minimum contact time of 15 s, regardless of protein load (except rotavirus, which was tested without protein load). Pathogens are eradicated by the active moiety (non PVP-bound ‘free’ iodine) being released into solution from the PVP-I complex, penetrating the cell wall and inactivating cells by forming complexes with amino acids and unsaturated fatty acids, resulting in impaired protein synthesis and alteration of cell membranes . This basic mechanism of action leads to strong microbicidal activity expressed by multiple modes of action that include the disruption of microbial metabolic pathways, as well as destabilisation of the structural components of cell membranes, causing irreversible damage to the pathogen .
The results of this study suggest that the use of PVP-I gargle/mouthwash may be a useful protective measure against oral and respiratory tract infections. Indeed, following the H1N1 swine flu outbreak in 2009, Japan’s Ministry of Health, Labour and Welfare recommended daily gargling as a protective hygiene measure to prevent upper respiratory tract infections (URTIs) , a practice supported by findings from studies that examined the role of gargling in both healthy individuals and those with frequent or persistent URTIs [14, 41, 42]. Limited clinical studies have been performed that used PVP-I gargle/mouthwash to reduce the incidence of respiratory infections in different settings. Shiraishi and Nakagawa showed a mean reduction rate in bacterial count immediately after gargling of 99.4% for PVP-I in volunteers (compared with 59.7% for CHG and 97.0% for CPC) and a significantly lower absence rate due to URTIs at a Japanese middle school where the use of PVP-I gargle was encouraged compared with schools where PVP-I gargle was not used . In patients with chronic respiratory diseases, gargling with PVP-I was found to reduce the episodes of infections with Pseudomonas aeruginosa, Staphylococcus aureus (including MRSA) and Haemophilus influenzae by half . Studies of prophylactic use of PVP-I gargle in patients requiring intubation have also shown significant reductions in oropharyngeal bacterial counts . Oral hygiene using PVP-I may be of particular benefit in certain patient groups such as immunocompromised patients at risk of prolonged virus shedding (which can increase the potential for resistance to antiviral drugs and for nosocomial transmission), patients with influenza to reduce the risk of secondary bacterial infection (that may appear, e.g., as otitis media in children and thus avoid the need for antibiotics) and possibly in hospitalised patients to prevent the spread of influenza during high season.
The safety profile of PVP-I is well established. In contrast to other antiseptic agents, PVP-I oral care products do not lead to any irritation or damage of the oral mucosa, even with prolonged use [44, 45]. Although measurable systemic iodine absorption may occur with the long-term use of PVP-I, its clinical manifestation as thyroid dysfunction is not very common .
A limitation of this work is that the clinical relevance of such in vitro test results remains unclear and needs to be supported by further investigations to evaluate the impact of gargling with PVP-I in real-life and clinical settings, although for ethical reasons, clinical studies involving highly infective and dangerous pathogens may not be feasible. Furthermore, our testing was limited to a few key respiratory microorganisms. We selected Streptococcus pneumoniae as the main cause of community-acquired pneumonia and meningitis and Klebsiella pneumoniae because, although not a common cause of respiratory tract infections, it is an emerging cause of multidrug-resistant nosocomial infection. In addition, these species represent both gram-positive and -negative bacteria. Although our study did not include other common bacterial pathogens causing pneumonia such as Staphylococcus aureus, Pseudomonas aeruginosa, Haemophilus influenzae and Acinetobacter baumannii, the efficacy of PVP-I against these pathogens has already been demonstrated in previous studies [16, 18, 32, 33, 41, 46].