* Energy sector lower; banks, utilities big losers
* Petsmart to be acquired by private equity consortium
* Indexes off: Dow 0.3 pct, S&P 0.3 pct, Nasdaq 0.7 pct (Updates to afternoon, changes byline)
By Caroline Valetkevitch
NEW YORK, Dec 15 (Reuters) - U.S. stocks were down slightly in volatile trading on Monday afternoon as investors assessed the impact of another big drop in oil prices.
U.S. crude fell 3.3 percent to settled at $55.91, hitting fresh 5-1/2 year lows. Both U.S. crude and Brent have fallen nearly 50 percent from highs in June.
The S&P 500 energy index bounced between positive and negative territory and was last down 0.5 percent, while financials and utilities were among the biggest negatives.
“What continues to be the focus for the markets is oil,” said Tim Ghriskey, chief investment officer of Solaris Group in Bedford Hills, New York.
“There was some buying, and we’ll see evidence of this ... buying on the dips.”
But he said oil is “where the heaviness is here going into year end. The positive impact from lower oil prices both in consumer and manufacturing won’t be felt immediately.”
S&P 500 traded below its 50-day moving average for the first time since Oct. 28.
At 2:31 p.m., the Dow Jones industrial average fell 44.5 points, or 0.26 percent, to 17,236.33, the S&P 500 lost 5.24 points, or 0.26 percent, to 1,997.09 and the Nasdaq Composite dropped 30.76 points, or 0.66 percent, to 4,622.84.
At its session low, the S&P 500 was down about 5 percent from its record intraday high hit earlier this month but up more than 8 percent from a low hit in October.
Exxon Mobil shares gained 0.5 percent.
Among the day’s economic numbers, U.S. manufacturing output recorded its largest increase in nine months in November as production expanded across the board, pointing to underlying strength in the economy. However, the New York Federal Reserve’s gauge of manufacturing turned negative in December for the first time in almost two years.
Shares of pet supply retailer PetSmart rose 4.2 percent after it agreed to be bought by a private equity consortium led by BC Partners Ltd for $8.7 billion, in the largest leveraged buyout of the year.
Declining issues outnumbered advancing ones on the NYSE by 2,262 to 805, for a 2.81-to-1 ratio on the downside; on the Nasdaq, 1,857 issues fell and 849 advanced for a 2.19-to-1 ratio favoring decliners.
The benchmark S&P 500 index was posting 10 new 52-week highs and 32 new lows; the Nasdaq Composite was recording 34 new highs and 159 new lows. (Additional reporting by Rodrigo Campos; Editing by Bernadette Baum, Nick Zieminski and Meredith Mazzilli)