Introduction

Acute sore throat is a common symptom of an upper respiratory tract infection (URTI), associated with inflammation of the pharynx, tonsils or nasopharynx [1]. The most frequent cause of acute sore throat is a viral infection, responsible for up to 80% of cases in adults [2]. Bacterial infections are estimated to cause 5–15% of acute sore throat cases in adults [3,4,5]. Streptococcus pyogenes (also known as group A β-hemolytic Streptococcus or GABHS) is the most common bacterial cause of acute sore throat [3, 5], although other bacterial species have been implicated, including Staphylococcus aureus [6], Moraxella catarrhalis [7], Haemophilus influenzae [8] and Fusobacterium necrophorum [9].

Antibiotics continue to be overprescribed for acute sore throat and are often unnecessary and ineffective in this setting [10], contributing to the growing problem of antibiotic resistance [11]. Even when the cause of sore throat is bacterial, in most cases it will be self-limiting and improve without the need for antibiotics [3]. For most individuals, over-the-counter treatments, such as lozenges, represent the primary option for relief from the symptoms of acute sore throat [12]. Lozenges containing the antiseptic hexylresorcinol significantly reduced the symptoms of acute sore throat over a 2-h study period in a placebo-controlled trial [13]. Furthermore, a concentration-dependent numbing effect with hexylresorcinol lozenges has been reported in healthy volunteers [14]. These effects are likely in part due to its local anesthetic activity, achieved through blocking voltage-gated neuronal sodium channels [15]. Hexylresorcinol has also demonstrated antiviral effects against species associated with URTIs [16] or known to cause acute sore throat [17].

In vitro studies have found that hexylresorcinol has antibacterial activity against a range of species when in solution and when embedded in biopolymer composite films [18, 19]. The antibacterial effects of hexylresorcinol in vivo may be mediated through several mechanisms including reducing bacterial adherence to the pharynx, inhibiting bacterial biofilm formation, disrupting bacterial cell chain formation, and modifying cell surface hydrophobicity [20]. However, no published studies have addressed whether hexylresorcinol lozenges have antibacterial activity against organisms implicated in acute sore throat.

This study determined the in vitro bactericidal activity of hexylresorcinol lozenges against a range of medically relevant oropharyngeal organisms associated with acute sore throat.

Main text

Methods

Test samples

Hexylresorcinol 2.4 mg lozenges (Strepsils Extra Honey and Lemon lozenges; Reckitt Benckiser, Slough, UK) were dissolved at 44 ± 1 ℃ into 5 mL of artificial saliva medium as described previously [21].

Test organisms

S. aureus (NCTC7445, Public Health England, Salisbury, UK), S. pyogenes (NCTC12696, Public Health England), M. catarrhalis (NCTC3622, Public Health England), H. influenzae (NCTC4842, Public Health England) and F. necrophorum (NCTC12238, Public Health England) were cultured as described previously [21].

Bactericidal assay

The bactericidal assay was performed using a method similar to that described previously [21]. Briefly, inoculum suspensions prepared in triplicate for each test organism, at approximately 108 colony-forming units (cfu)/mL in saline, were mixed with hexylresorcinol test sample (4.9 mL). Bactericidal activity was assayed after 1-, 5-, 10- and 30-min exposure times by combining sample/inocula mixture (1 mL) with neutralizing diluent (9 mL). Serially-diluted solutions were incubated on suitable agar medium for at least 3 days. Bactericidal activity was also assayed at the 30-min time point for inoculum cultures (0.1 mL) for each test organism mixed with a positive control sample of artificial saliva medium (4.9 mL). Mean log reduction (in cfu/mL) for test samples was calculated for each organism and time point (average of three triplicates) relative to test controls.

Results

Test control counts demonstrated that the test method and media did not affect the survival of the organisms. Following test sample inoculation, evidence of bactericidal activity was recorded at the 1-min time point for all organisms (Table 1, Fig. 1). For S. aureus, M. catarrhalis, H. influenzae and F. necrophorum, the decrease at 1 min exceeded 3log10 (99.9% reduction) (Table 1). For S. pyogenes, a 2.9log10 reduction was seen at 1 min and a greater than 3log10 reduction was recorded by 5 min (Table 1). For all test organisms, the lower limit of detection in the bactericidal activity assay was reached at the 30-min time point (Table 1).

Table 1 Bactericidal activity of hexylresorcinol lozenges
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Fig. 1
figure 1

Bactericidal activity of hexylresorcinol lozenges. Bactericidal activity, defined as a decrease in bacterial count (log10 cfu/mL), was measured against five common oropharyngeal organisms over a 30-min period. Only activity measurements up to 10 min are shown. *Average of the three test replicates. cfu colony-forming units

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Discussion

Hexylresorcinol lozenges demonstrated rapid antibacterial activity against a broad range of organisms implicated in acute sore throat, including Gram-positive species (S. aureus and S. pyogenes) and Gram-negative species (M. catarrhalis, H. influenzae and F. necrophorum). Across all test organisms, bactericidal activity was seen from the 1-min time point. For S. aureus, M. catarrhalis, H. influenzae and F. necrophorum, the decrease at 1 min exceeded 3log10 (99.9% reduction). For S. pyogenes, the most common cause of bacterial acute sore throat [3], the decrease was 2.9log10 at 1 min and greater than 3log10 reductions were recorded by 5 min. To the best of our knowledge, these are the first published data indicating that hexylresorcinol-containing lozenges have bactericidal activity in vitro.

The study was designed to simulate the clinical setting as far as possible, including the time taken for a lozenge to dissolve in the mouth (mean ± standard deviation: 6.77 ± 2.01 min) [22]. Furthermore, the method was designed to replicate the expected concentration of hexylresorcinol that would be achieved when a lozenge is dissolved in the mouth, assuming a volume of 5 mL of saliva.

The findings in this study are consistent with previously reported antibacterial effects of hexylresorcinol in solution against a range of organisms, including Streptococcus spp. and S. aureus [18]. In addition, these data are in line with studies of other over-the-counter acute sore throat treatments. In a similar in vitro study, Matthews et al. (2018) reported that lozenges containing 0.6 mg amylmetacresol and 1.2 mg 2,4-dichlorobenzyl alcohol had broad antibacterial activity against a similar range of oropharyngeal organisms associated with acute sore throat. Specifically, reductions in bacterial counts exceeded 99.9% by 1 min for S. pyogenes, H. influenzae, F. necrophorum and A. haemolyticum, by 5 min for M. catarrhalis and S. dysgalactiae and by 10 min for S. aureus [21]. The antibacterial effects of hexylresorcinol lozenges reported here add to existing knowledge of their activity, which include numbing effects [13, 14] and antiviral activity [16, 17], resulting in relief of sore throat symptoms [13].

In conclusion, hexylresorcinol lozenges demonstrated bactericidal activity against medically relevant oropharyngeal organisms associated with acute sore throat from 1 min and achieved a > 99.9% reduction in cfu/mL for all test organisms within 5 min, which is in line with the duration for a lozenge to dissolve in the mouth. Thus, hexylresorcinol lozenges represent an effective over-the-counter treatment option for acute sore throat, offering rapid antibacterial, antiviral and local anesthetic effects, and may help to avoid unnecessary antibiotic prescribing, which is associated with the development of antibiotic resistance.

Limitations

  • In vitro models cannot precisely replicate how lozenges will act in a patient’s throat. Therefore, additional studies may be needed to confirm the antibacterial activity of hexylresorcinol lozenges in a clinical setting.

  • The lower limit of detection of the bactericidal activity assay in this study was rapidly reached following addition of hexylresorcinol: by 1-min exposure for H. influenzae and F. necrophorum, 5-min exposure for S. aureus and M. catarrhalis and 10-min exposure for S. pyogenes. The use of more sensitive analytical methods may have allowed for a greater limit of detection.