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Background: Acute tonsillitis and peritonsillar abscess (PTA) are common infections in primary care and otorhinolaryngology. Uncertainties remain regarding microbial aetiology, optimal diagnosis, and treatment. This thesis clarifies the roles of key bacterial pathogens, evaluates diagnostic methods, and supports evidence-based clinical decision-making. 

Materials & Methods: Three studies were conducted. Study I included healthy individuals; Study II included patients with tonsillitis or PTA. Tonsillar swabs and peritonsillar pus were analysed by culture and PCR for Streptococcus pyogenes (GAS), Streptococcus dysgalactiae subspecies equisimilis (SDSE), Fusobacterium necrophorum (FN), and Arcanobacterium haemolyticum (AH). Serum CRP and calprotectin were measured to assess inflammatory response and diagnostic performance. Study III included PTA patients and retrospectively examined risk factors for recurrence and tonsillectomy outcomes. 

Results: Project I included 217 healthy adolescents and young adults (16–25 years). Tonsillar swabs showed 25 % carried one or more target species: GAS and FN were each present in 10% of individuals, SDSE in 8 %, and AH in 1 %. GAS and FN had significantly higher detection by PCR than culture, mainly due to individuals where semi-quantitative PCR indicated a low bacterial load in the initial samples.

Project II included 113 acute-tonsillitis patients (Centor score 3–4) and 70 PTA patients. Culture and PCR had similar detection for all targets except AH, detected more often by PCR in tonsillitis. 

Tonsillitis patients had a target bacterial species in 79 % of cases: GAS 63 %, FN 12 %, SDSE 7 %, AH 7 %. Patients with GAS had higher CRP and calprotectin levels than patients with other bacteria, or no bacteria.

In PTA, GAS occurred in 35 %, FN 22 %, SDSE 6 %, AH 2 %. PTA patients with FN had significantly higher CRP and calprotectin than others.

Project III included 292 consecutive PTA patients, of whom 18 underwent immediate, quinsy tonsillectomy. Of the remaining 274 who underwent pus drainage by aspiration or incision, 19 % experienced a recurrence. Cox regression identified young age as a risk factor for recurrence, while sex and a previous history of PTA were not significant. Among the youngest patients (15–24 years), PTA recurred in 28 %, most often within 30 days, and 47 % of them underwent tonsillectomy during the study period (for any indication). Older patients (>30 years) had few recurrences (4 %) and few tonsillectomies (15 %). Tonsillectomy effectively eliminated recurrence, as none of the 101 operated patients experienced PTA recurrence after surgery.

Conclusion: Asymptomatic carriage of GAS, SDSE, and FN is common among adolescents and young adults. GAS predominated in tonsillitis, while other pharyngeal bacterial pathogens were detected at low rates. CRP and calprotectin increase in GAS tonsillitis but add no diagnostic value. GAS and FN predominated in PTA, with the highest inflammatory biomarker levels observed in FN cases. The risk of PTA recurrence is strongly age-dependent; early tonsillectomy should be considered in the youngest patients.