Penelitian ini bertujuan untuk memperoleh informasi mengenai pengaruh variasi mediumpertumbuhan terhadap pembentukan miselium aerial dan aktivitas antimikroba delapanisolat rare Actinobacteria termofilik dari tanah di sekitar geiser Cisolok, Jawa Barat.Pengujian pertumbuhan, pembentukan miselium aerial, dan aktivitas antimikrobadilakukan dengan menumbuhkan isolat pada medium ISP 1 agar, ISP 1 gellan gum, ISP2 agar, ISP 2 gellan gum, ISP 3 agar, ISP 3 gellan gum, Bennett’s agar, Bennett’s gellangum, Minimal (Mm) 3 agar, Mm 3 gellan gum, 2% agar, dan 2% gellan gum. Isolatkemudian diinkubasi pada suhu 45 °C selama 7 dan 14 hari. Konfirmasi suhupertumbuhan menunjukkan 2 isolat dapat tumbuh hingga suhu 45 °C dan 6 isolat dapattumbuh hingga 50 °C. Hasil pengujian variasi medium pertumbuhan menunjukkan semuaisolat rare Actinobacteria dapat menghasilkan miselium substrat pada semua medium.Hasil pengamatan setelah inkubasi selama 7 hari pada suhu inkubasi 45 °C menunjukkanisolat-isolat tersebut dapat menghasilkan miselium aerial pada medium ISP 1 agar (2isolat), Mm 3 agar (3 isolat), 2% agar (5 isolat), dan 2% gellan gum (5 isolat). Hasilpengamatan setelah inkubasi selama 14 hari menunjukkan isolat-isolat tersebut dapatmenghasilkan miselium aerial pada medium ISP 1 gellan gum (2 isolat), ISP 2 agar (1isolat), ISP 2 gellan gum (3 isolat), ISP 3 agar dan gellan gum (2 isolat), Mm 3 agar 3isolat, dan Mm 3 gellan gum (3 isolat). Hasil pengujian aktivitas antibakteri menunjukkanisolat SL3-2-R-11 yang ditumbuhkan pada medium ISP 3 gellan gum dan SL3-1-R-7yang ditumbuhkan pada Bennett’s agar selama 7 hari dapat menghambat pertumbuhanStaphylococcus aureus. Hasil pengujian aktivitas antikhamir menunjukkan isolat SL3-2-R-11 yang ditumbuhkan pada medium ISP 3 gellan gum dan SL3-1-R-7 pada mediumBennett’s agar selama 14 hari dapat menghambat pertumbuhan Candida albicans. Hasilpengujian aktivitas antifungi menunjukkan tidak ada isolat yang dapat menghambatpertumbuhan Aspergillus flavus.

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This study aims to obtain information about the effect of growth medium variations onthe formation of aerial mycelium and antimicrobial activity of eight thermophilic rareActinobacteria isolates from the soil around Cisolok geyser, West Java. The ability togrow at various media, aerial mycelium formation, and antimicrobial activity were carriedout by growing isolates on medium ISP 1 agar, ISP 1 gellan gum, ISP 2 agar, ISP 2 gellangum, ISP 3 agar, ISP 3 gellan gum, Bennett’s agar, Bennett’s gellan gum, Minimum (Mm)3 agar, Mm 3 gellan gum, 2% agar, and 2% gellan gum. The isolates were then incubatedat 45 oC for 7 and 14 days. Growth test at various temperatures showed that two isolatescould grow at a temperature of 45 oC and six isolates could grow up to 50 oC. The resultsof the growth medium variation test showed that all rare Actinobacteria isolates couldproduce substrate mycelium in all mediums. Observations after incubation for 7 days at45 °C showed that these isolates could produce aerial mycelium on ISP 1 agar medium(2 isolates), Mm 3 agar (3 isolates), 2% agar (5 isolates), and 2% gellan gum (5 isolates).Observations after incubation for 14 days showed that these isolates could produce aerialmycelium on the medium ISP 1 gellan gum (2 isolates), ISP 2 agar (1 isolate), ISP 2gellan gum (3 isolates), ISP 3 agar and gellan gum (2 isolates), Mm 3 agar 3 isolates, andMm 3 gellan gum (3 isolates). The results of antibacterial activity test showed that isolatesSL3-2-R-11 grown on ISP 3 gellan gum and SL3-1-R-7 grown on Bennett’s agar for 7days could inhibit the growth of Staphylococcus aureus. The antifungal activity test ofisolates SL3-2-R-11 grown on ISP 3 gellan gum medium and SL3-1-R-7 on Bennett’sagar for 14 days showed inhibition towards Candida albicans. Meanwhile, all isolates didnot show antifungal activity against Aspergillus flavus