生物氣膠之採樣遮蔽效應對計數效率之影響

The Masking Effect on Bioaerosol Colony Counting

生物氣膠採樣分析上由於有計數的步驟，常會因採樣濃度過高而造成計數誤差的問題。由理論及以往一些實驗得知：若是採樣密度過高，則因菌落生長時互相重疊而造成計數結果的低估；若是採樣濃度過低，則又有樣本代表性不足的問題。本研究嘗試以電腦模擬的方式，探討在不同參數下遮蔽效應對菌落計數效率的影響，模擬參數包括：菌落大小（直徑；1mm、2mm、5mm、8mm）、培養皿表面的菌落密度、GSD（對數常態分佈的幾何標準偏差）、以及鑑別率等。結果發現計數的精確度會隨著培養皿表面菌落密度的增加而降低。對直徑分別為 2、5、8 mm的菌落來說，若其培養皿表面密度不超過1290、200及71 count/ plate，則其計數效率而言可達95％（保守推估人眼及觀測系統鑑別率為0.8時）；如果想提高計數效率到達97％，則前述各粒徑的上限值減為900、140、50 count/plate。至於菌落密度對計數效率的影響則以大菌落為嚴重，其計數效率下降幅度隨著菌落直徑的增加而增大。此外當GSD值等於1時，覆蓋率最低，也就是說遮蔽效應的影響最小，所以其計數效率最高，而隨著GSD值繼續的增加，覆蓋率也會隨之增加，計數效率也就逐漸降低，且此現象與鑑別率高低無關。

The studies on the error due to overcrowding of the colonies have concluded that the bias did not become significant until about 300 colonies/plate were present. The resolution of 1000 x 1000 was found accurate enough to represent the actual size of the colony. The capability of distinguishing overlapping colonies, defined as resolution index, the covering fraction, the colony surface density, the colony size and distribution were the primaty parameters in this study. The counting efficiency was found to consistently decrease with increasing surface density, decreasing resolution index and increasing colony size. To have a minimum counting efficiency of 95%, the colony count should not exceed 1290, 200, and 71 count/plate, for the colony size of 2, 5 and 8 mm, given the resolution index of 0.8. When GSD=1, the covering fraction is found to be the smallest; therefore, counting efficiency becomes the highest. However, increasing GSD will result in higher covering fraction, thus, lower counting efficiency.