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Introduction
Streptococcus mitis is a commensal bacterium belonging to the viridans group streptococci (VGS). While primarily found in the oropharynx, it also resides on the skin, in the gut, and in the female genital tract. As an opportunistic pathogen, it can cause a range of invasive diseases, particularly in immunocompromised patients, such as those with malignancies (Shelburne et al. 2014), chemotherapy, and neutropenia (Marron 2000). In recent years, the number of S. mitis infection cases has gradually increased. It has been confirmed to cause bloodstream infections, infective endocarditis, meningitis (Fukayama et al. 2022), pneumonia, odontogenic infections, peritonitis (Kanjanabuch et al. 2024), and abscess-related conditions (Prod’homme et al. 2021; Zhang et al. 2024).
Among the oral streptococci, S. mitis is a leading cause of infective endocarditis and bacteremia (Kalizang’oma et al. 2024). In studies by the Health Protection Agency (HPA) in the UK, the rate of bacteremia caused by S. mitis exceeded that of group A or Group B streptococci (Kalizang’oma et al. 2024). Patients tend to become infected with their own commensal strains, and the mortality rate from S. mitis bacteremia ranges from 6 to 30% (Thielemans et al. 2020). S. mitis can escape from this oropharynx niche and cause a variety of infectious complications, which uses a variety of strategies to colonize the human oropharynx effectively. These include expression of adhesins, immunoglobulin A proteases (De Paolis et al. 2007) and toxins (Lessa et al. 2018), and modulation of the host immune system (Tian et al. 2019). At the same time, S. mitis genomes may possess a range of pneumococcal virulence genes (Rasmussen et al. 2017), including those encoding the capsule (Lessa et al. 2018), IgA1 protease, pneumolysin, and autolysin compared to Streptococcus pneumoniae (Denapaite et al. 2010). So the researchers put out the point about S. mitis walking the line between commensalism and pathogenesis (Mitchell 2011). With the widespread use of antimicrobial agents, the resistance patterns of S. mitis have also evolved. Among isolates from UK and Ireland bloodstream infection and infective endocarditis cases with phenotypic Minimum Inhibitory Concentration data, 23.3% and 6.2% of the isolates were non-susceptible to penicillin and amoxicillin, respectively (Kalizang’oma et al. 2024).
Given the potential virulence of S. mitis as well as drug resistance, this study analyzed the clinical distribution characteristics and antimicrobial resistance trends of 249 S. mitis isolates collected over five consecutive years from a tertiary hospital, aiming to provide insights for clinical treatment and hospital infection control.
Experimental
Materials and Methods
Study design and case ascertainment
We performed a retrospective analysis of patients with S. mitis detection admitted to a tertiary hospital in Yancheng, Jiangsu, from 1 January 2020 to 31 December 2024. S. mitis infection cases were clinically collected, and duplicate strains detected in the same site of the same patient were excluded. Cases were excluded if: i) S. mitis was detected in sputum/oral/respiratory samples without compatible clinical symptoms; ii) non-sterile urine cultures grew S. mitis with ≥ 2 coexisting organisms; or iii) isolates were classified as contaminants per microbiological standards.
Data collection
Data were obtained from case notes and electronic hospital records through the Apricot Grove Nosocomial Sensory Surveillance System. Demographic details, including age at diagnosis, gender, department, and laboratory parameters, were collected.
Determination of infection and colonization
The criteria for distinguishing infection from colonization were based on a combination of clinical, microbiological, and laboratory findings. In ambiguous cases, inflammatory markers and histopathology were used to support diagnosis. The infection status is assessed by hospital infection control specialists based on the “Diagnostic Criteria for Nosocomial Infections (Trial)” issued by the Ministry of Health in 2001. Each case of S. mitis infection is reviewed and evaluated by two personnel for verification. Infections are classified into three categories: CAI which defined as infections present at admission or diagnosed within 48 hours, with no prior healthcare exposure within 30 days; HAI which defined as infections occurring ≥ 48 hours after admission, in patients with no evidence of incubation at admission and Colonization which defined as bacterial growth in non-sterile sites without clinical symptoms or repeated positivity.
Bacterial identification and antimicrobial susceptibility testing
Clinical specimens were streaked onto 5% sheep blood agar plates (Oxoid, UK) and incubated at 37°C for 24–48 hours under 5% CO2. Colonies exhibiting α-hemolysis (greenish discoloration) were selected for further identification by using the VITEK® 2 Compact bacterial identification system (bioMérieux, France) and the VITEK® matrix-assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF) (bioMérieux, France). The identification result is considered reliable when the confidence score reaches 99.9%. Optochin testing was also used to differentiate S. pneumoniae from other streptococci. Antimicrobial Susceptibility Testing (AST) is performed on Mueller–Hinton agar supplemented with 5% sheep blood by using the Kirby–Bauer disk diffusion method (KB) (Oxoid ™, Thermo Fisher Scientific Inc., USA). The CLSI M100 32rd ed. (2022) guidelines for VGS group was used to determine the susceptibility to antimicrobial agents (cefepime/30μg, ceftriaxone/30μg, erythromycin/15μg, clindamycin/2μg, levofloxacin/5μg, linezolid/30μg, vancomycin/30μg, penicillin G/10 IU).
Statistical Analysis
All statistical analyses were conducted using IBM SPSS Statistics for Windows v25.0 (IBM Corp., USA). Measurement data were expressed as counts or percentages, and the χ2 test was used for analysis. A p-value of less than 0.05 was considered statistically significant.
Results
Trends in the detection of S. mitis between 2020 and 2024
During the study period, a total of 322 S. mitis detection cases were recorded. According to the criteria, 249 cases of S. mitis were detected and included, while 73 were excluded. Among these included cases, the number of S. mitis isolates was increasing, and the detection rate of S. mitis cases per 100,000 from years 2020 to 2024 showed an upward trend (χ2 = 91.163, p < 0.001), as shown in Table I.
Trends in detection rate of Streptococcus mitis from 2020 to 2024.
Year
Annual patients volume
Cases
Rate (‱)
2020
65,378
16
2.45
2021
67,020
17
2.54
2022
70,952
36
5.10
2023
78,830
66
8.37
2024
82,557
114
13.81
Total
364,737
249
6.83
Clinical characteristics of patients with detected S. mitis
We analyzed the 249 included cases by patient gender, age, infection type, and department (Table II). The majority of the study population was male, with a percentage of 59.04% (147/249). Cases were detected across all age groups, with the lowest proportion observed in patients > 80 years old. In terms of infection types, colonization was the most common, followed by CAI and HAI. Among the departments with detected S. mitis, the top three were Pediatrics, Urology, and Stomatology (Table II).
Clinical characteristics of patients with Streptococcus mitis infection from 2020 to 2024.
Clinical characteristics
Parameters
Cases
Rate (%)
Sex
Male
147
59.04
Female
102
40.96
Age in years
≤ 20
65
26.10
21~40
27
10.84
41~60
68
27.31
61~80
84
33.74
≥ 81
5
2.01
Type of infection
HAI
21
8.43
CAI
112
44.98
Colonization
116
46.59
Department
Pediatrics
55
22.09
Urinary
49
19.68
Stomatology
29
11.65
ICU
18
7.23
Radiation
15
6.02
Respiratory medicine
10
4.02
Orthopedics
10
4.02
General surgery
9
3.61
Endocrine
8
3.21
Cardiothoracic surgery
8
3.21
Gastroenterology
9
3.61
Neonatology
5
2.01
Hematology
5
2.01
Others
18
7.23
Notably, both CAI and colonization cases were predominant across all age groups. The 40–60 and 60–80 age groups showed higher proportions of combined HAI and CAI cases. Interestingly, the 61–80 age range represented the highest proportion within any single infection type category. Among patients < 20 years old, CAI was the most prevalent infection type, which aligns with previous reports (Table III).
Analysis of different infection types and patient age, specimen source, and antimicrobial resistance.
Characteristics
Infection type
HAI (21)
CAI (112)
Colonization (116)
Parameters
Cases/%
Cases/%
Cases/%
Ages
≤ 20
2/9.52
30/26.79
33/28.44
21~40
2/9.52
10/8.93
15/12.93
41~60
6/28.58
35/31.25
27/23.28
61~80
9/42.86
36/32.14
39/33.62
≥ 81
2/9.52
1/0.89
2/1.72
Source
Skin and soft tissue
4/19.05
18/16.70
36/31.03
Secretion bronchoalveolar lavage fluid
1/4.76
25/22.3
32/27.59
Urine
4/19.05
14/12.5
33/28.44
Blood
10/47.62
34/30.35
0
Punctate fluid
0
6/5.36
8/6.90
Pus
1/4.76
4/3.57
3/2.59
Bile
0
2/1.79
4/3.45
Pleural effusion
0
5/4.46
0
Ascites
1/4.76
4/3.57
0
Resistance
Cefepime
5/23.81
11/9.82
13/11.21
Ceftriaxone
11/52.38
13/11.61
15/12.93
Erythromycin
18/85.71
48/42.86
93/80.17
Clindamycin
15/71.43
43/38.40
55/47.41
Levofloxacin
10/47.62
30/26.79
38/32.76
Linezolid
0
0
0
Vancomycin
0
0
0
Penicillin G
19/90.48
40/35.71
42/36.21
The distribution of specimen sources in S. mitis
After evaluation of microbiological data and the clinical condition of patients, a total of 249 patient specimens, including respiratory tract infections, urinary tract infections, skin and soft tissue infections, bloodstream infections, and others, were analyzed. By the site of infection, the most common focus of infections was skin and soft tissue infections and bronchoalveolar lavage fluid (BALF) infections, followed by urinary tract infections, and bloodstream infections. In total, the proportion of sterile body fluids (including blood, pleural fluid, ascites, and bile) was 32.93%. Although BALF is theoretically a sterile specimen, it may become contaminated by normal upper respiratory tract flora during the collection process (Table IV).
Specimen sources of Streptococcus mitis isolates during 2020–2024.
Specimen source
Isolates (n = 249)
Rate (%)
Skin and soft tissue secretion
58
23.30
bronchoalveolar lavage fluid
58
23.30
Urine
51
20.48
Blood
44
17.67
Punctate fluid
14
5.62
Pus
8
3.21
Bile
6
2.41
Pleural effusion
5
2.01
Ascites
5
2.01
Next, we analyzed specimen sources across different infection types. In the HAI group, bloodstream infections (BSI) predominated. The CAI group showed higher isolation rates from both bloodstream and BALF, while the colonization group was primarily associated with skin, BALF, and urine specimens (Table III). To further elucidate epidemiological patterns, we examined the relationship between patient age and specimen sources. Our analysis revealed that in patients < 20 years old, BALF had the highest S. mitis isolation rate, potentially linked to the frequent use of bronchoscopy in pediatric care and among 41–60 and 61–80-year-olds groups, sterile body fluids yielded significantly higher isolation rates compared to other groups (Table V).
Specimen sources of Streptococcus mitis isolates in different ages groups.
Specimen source
≤ 20
21~40
41~60
61~80
≥ 81
Skin and soft tissue secretion
14
10
10
23
1
bronchoalveolar lavage fluid
24
3
13
17
1
Urine
10
4
16
19
2
Blood
9
3
16
16
0
Punctate fluid
5
1
2
5
1
Pus
1
2
3
2
0
Bile
1
3
2
0
0
Pleural effusion
1
0
3
1
0
Ascites
0
1
3
1
0
Resistance rates of Streptococcus mitis to antimicrobial agents during 2020–2024.
Antimicrobial agents
2020 (n = 16)
2021 (n = 17)
2022 (n = 36)
2023 (n = 66)
2024 (n = 114)
χ2
p
R (%)
R (%)
R (%)
R (%)
R (%)
Cefepime
43.75
17.65
11.11
12.12
6.14
10.644
0.001
Ceftriaxone
25.00
29.41
22.22
22.72
6.14
7.399
0.007
Erythromycin
68.75
88.23
83.33
71.21
37.72
6.586
0.010
Clindamycin
50.00
100.00
63.88
48.48
28.95
8.514
0.004
Levofloxacin
50.00
52.94
30.55
42.42
19.30
5.564
0.018
Linezolid
0
0
0
0
0
–
–
Vancomycin
0
0
0
0
0
–
–
Penicillin G
55.55
41.17
72.22
46.96
24.56
6.608
0.010
Resistance rates of Streptococcus mitis to antimicrobial agents in different age groups.
Antimicrobial agents Age
≤ 20
21~40
41~60
61~80
≥ 81
Cefepime
0
1
8
16
4
Ceftriaxone
4
5
10
15
5
Erythromycin
13
20
48
73
5
Clindamycin
9
18
39
44
3
Levofloxacin
2
11
26
36
3
Linezolid
0
0
0
0
0
Vancomycin
0
0
0
0
0
Penicillin G
10
16
31
39
5
Resistance rates of S. mitis to antimicrobial agents between 2020 and 2024
According to the rules of CLSI (2000), the Kirby–Bauer disk diffusion method was used to monitor the sensitivity of S. mitis to eight antimicrobial agents. In general, there were no S. mitis isolates that showed resistance to vancomycin and linezolid (Table IV). In terms of years, analysis of the resistance trend of S. mitis from years 2020–2023 remained stable in its resistance to six antimicrobial agents except for linezolid and vancomycin (p < 0.05), which especially exhibited the lowest level in 2024 (Fig. 1). We speculate the observed reduction in antimicrobial resistance rates of 2024 may result from multiple factors: i) improved identification accuracy through updated MALDI-TOF MS technology, reducing S. mitis misclassification; ii) decreased pathogen transmission due to enhanced respiratory protection measures during the COVID-19 pandemic; and iii) increased microbiological surveillance through more frequent specimen testing.
Fig. 1.
The resistance of Streptococcus mitis to antimicrobial agents from 2020 to 2024.
As evidenced in Table III, the HAI group exhibited the highest overall antibiotic resistance rates, with penicillin G resistance reaching 90.48%. The resistance patterns in CAI and colonization groups were remarkably similar. These findings suggest that extensive antibiotic use in hospital settings contributes significantly to increased drug resistance in S. mitis. Furthermore, Table V reveals that the younger age group (< 20 years) showed lower antibiotic resistance compared to older age groups (except those > 80 years). This age-related resistance pattern may be closely associated with differential antibiotic exposure across age populations.
Discussion
S. mitis is an opportunistic pathogen and a common cause of nosocomial infections (Talari Sree et al. 2024). As early as 2002, it was reported that the detection rate of VGS from the blood of cancer patients with neutropenia had increased significantly over the past 10–15 years (Tunkel and Sepkowitz 2002). The researcher also reported that infections caused by S. mitis were more likely to be found in cancer patients with severe clinical symptoms (Shelburne et al. 2014). Research showed that S. mitis was one of the most important pathogenic microorganisms in pediatric bloodstream infections (BSI), with 34% of pediatric BSI patients presenting with febrile neutropenia and half of the patients having hematological diseases, malignancies or bone marrow transplantation (Reilly and Lange 2007; Basaranoglu et al. 2019; Thielemans et al. 2020). In this study, the clinical detection rate of S. mitis infection did not show a significant increase in specimens from oncology and hematology departments. However, it was relatively higher in pediatrics compared to other departments. This may be due to the low number of microbiological tests performed in the hematology department and patients with severe disease being transferred to higher hospitals for treatment. Data from this study also showed that among the 249 cases of S. mitis infections, 21 cases (8.43%) were HAP infections, indicating that S. mitis is also an important pathogen causing nosocomial infections. The study found a higher detection rate of this bacterium in esophageal cancer patients, likely due to the displacement of oral bacteria following long-term placement of gastric and duodenal feeding tubes (Narikiyo et al. 2004). Therefore, postoperative nosocomial infections in thoracic surgery patients should also be given high attention by medical staff.
Table I shows that the number of S. mitis cases increased each year, partly due to the increase in the number of infections. Meanwhile, the application of MALDI-TOF technology has shown significant advantages in the identification of S. mitis. Accurate identification of VGS has always been a challenge with traditional and molecular methods. Currently, there are three main methods for identifying S. mitis: phenotypic testing, MALDI-TOF and whole genome sequencing (Imai et al. 2020). However, several previous studies have questioned the ability of MALDI-TOF MS to identify VGS at the species level, in particular the misidentification of S. mitis as S. pneumoniae (Sadowy et al. 2020). The updated MALDI-Biotyper database, utilizing new algorithms, performs better, providing accurate information at the species level and no misidentification of S. mitis population strains (Harju et al. 2017; Pan et al. 2023; Tsai-Wen Wan et al. 2023;). Now researchers are developing a new pre-treatment of Streptococcus for mass spectrometry to improve accuracy (Nix et al. 2021).
The prophylactic use of antimicrobial agents against VGS could reduce the incidence of serious infections, which may raise concerns about drug resistance in VGS and other microorganisms. Additionally, mortality in neutropenic patients with penicillin-resistant streptococcal bacteremia may be higher than in those infected with penicillin-sensitive strains (Zhou et al. 2024). The resistance trends of 249 strains of S. mitis from 2020 to 2024 in this study did not show an increase in resistance to cefepime, ceftriaxone, erythromycin, clindamycin, levofloxacin, and penicillin G, while no resistance was observed for linezolid and vancomycin, consistent with the literature (Reilly and Lange 2007; Basaranoglu et al. 2019) (Fig1). Considering the differences in antimicrobial usage patterns across regions, the resistance of S. mitis to antimicrobial agents also varies. The resistance rates of this bacterium to cefepime (11.64%) and ceftriaxone (15.66%) in this study were significantly lower than those for VGS (34% for cefepime and 42% for ceftriaxone) (Marron et al. 2001), while the resistance rate to penicillin G was close to that reported by Basaranoglu et al. (2019) ST (45%). The primary mechanism of resistance may be due to excessive exposure of the bacteria to β-lactam antibiotics (Ergin et al. 2011). Additionally, the high resistance rate to levofloxacin may be related to the common use of quinolones in treating urinary tract infections (Dinani et al. 2009; Zhang et al. 2023). The increasing resistance of S. mitis should be taken seriously in clinical practice, as cases of resistance to penicillin, amoxicillin, quinolones (Zhang et al. 2023), and even vancomycin (Krcmery et al. 1996) have been reported.
Currently, there is limited data on the resistance of S. mitis to antimicrobial agents, making it even more necessary to monitor clinical susceptibility data to better guide clinical treatment closely. In summary, the detection rate of S. mitis in this study increased annually. While the overall trend of antimicrobial resistance remained relatively stable, the rational selection of antimicrobial agents and monitoring of resistance trends remain crucial.
